GCC 11 Release Series
Changes, New Features, and Fixes

This page is a "brief" summary of some of the huge number of improvements in
GCC 11. You may also want to check out our Porting_to_GCC_11 page and the full
GCC_documentation.

Caveats


* The default mode for C++ is now -std=gnu++17 instead of -std=gnu++14. Note
  that C++17_changes_to_template_template_parameter_matching can be disabled
  independently of other features with -fno-new-ttp-matching.
* When building GCC itself, the host compiler must now support C++11, rather
  than C++98. In particular bootstrapping GCC 11 using an older version of GCC
  requires a binary of GCC 4.8 or later, rather than of GCC 3.4 or later as was
  the case for bootstrapping GCC 10.
* Naming and location of auxiliary and dump output files changed. If you
  compile multiple input files in a single command, if you enable Link Time
  Optimization, or if you use -dumpbase, -dumpdir, -save-temps=*, and you
  expect any file other than the primary output file(s) to be created as a side
  effect, watch out for improvements and a few surprises. See the_patch,
  particularly its textual description, for more details about the changes.
* -gsplit-dwarf no longer enables debug info generation on its own but requires
  a separate -g for this.
* The libstdc++ configure option --enable-cheaders=c_std is deprecated and will
  be removed in a future release. It should be possible to use --enable-
  cheaders=c_global (the default) with no change in behaviour.
* The front end for compiling BRIG format of Heterogeneous System Architecture
  Intermediate Language (HSAIL) has been deprecated and will likely be removed
  in a future release.
* Some short options of the gcov tool have been renamed: -i to -j and -j to -H.


General Improvements


* ThreadSanitizer improvements to support alternative runtimes and
  environments. The Linux_Kernel_Concurrency_Sanitizer_(KCSAN) is now
  supported.

  o Add --param tsan-distinguish-volatile to optionally emit instrumentation
    distinguishing volatile accesses.
  o Add --param tsan-instrument-func-entry-exit to optionally control if
    function entries and exits should be instrumented.

* In previous releases of GCC, the "column numbers" emitted in diagnostics were
  actually a count of bytes from the start of the source line. This could be
  problematic, both because of:

  o multibyte characters (requiring more than one byte to encode), and
  o multicolumn characters (requiring more than one column to display in a
    monospace font)

  For example, the character π ("GREEK SMALL LETTER PI (U+03C0)")
  occupies one column, and its UTF-8 encoding requires two bytes; the character
  🙂 ("SLIGHTLY SMILING FACE (U+1F642)") occupies two columns, and its
  UTF-8 encoding requires four bytes.
  In GCC 11 the column numbers default to being column numbers, respecting
  multi-column characters. The old behavior can be restored using a new option
  -fdiagnostics-column-unit=byte. There is also a new option -fdiagnostics-
  column-origin=, allowing the pre-existing default of the left-hand column
  being column 1 to be overridden if desired (e.g. for 0-based columns). The
  output of -fdiagnostics-format=json has been extended to supply both byte
  counts and column numbers for all source locations.
  Additionally, in previous releases of GCC, tab characters in the source would
  be emitted verbatim when quoting source code, but be prefixed with whitespace
  or line number information, leading to misalignments in the resulting output
  when compared with the actual source. Tab characters are now printed as an
  appropriate number of spaces, using the -ftabstop option (which defaults to 8
  spaces per tab stop).
* Introduce Hardware-assisted_AddressSanitizer support. This sanitizer
  currently only works for the AArch64 target. It helps debug address problems
  similarly to AddressSanitizer but is based on partial hardware assistance and
  provides probabilistic protection to use less RAM at run time. Hardware-
  assisted_AddressSanitizer is not production-ready for user space, and is
  provided mainly for use compiling the Linux Kernel.
  To use this sanitizer the command line arguments are:

  o -fsanitize=hwaddress to instrument userspace code.
  o -fsanitize=kernel-hwaddress to instrument kernel code.

* For targets that produce DWARF debugging information GCC now defaults to
  DWARF_version_5 (with the exception of VxWorks and Darwin/Mac OS X which
  default to version 2 and AIX which defaults to version 4). This can produce
  up to 25% more compact debug information compared to earlier versions.
  To take full advantage of DWARF version 5 GCC needs to be build against
  binutils version 2.35.2 or higher. When GCC is build against earlier versions
  of binutils GCC will still emit DWARF version 5 for most debuginfo data, but
  will generate version 4 debug line tables (even when explicitly given -
  gdwarf-5).
  The following debug information consumers can process DWARF version 5:

  o GDB 8.0, or higher
  o valgrind 3.17.0
  o elfutils 0.172, or higher (for use with systemtap, dwarves/pahole, perf and
    libabigail)
  o dwz 0.14

  Programs embedding libbacktrace are urged to upgrade to the version shipping
  with GCC 11.
  To make GCC 11 generate an older DWARF version use -g together with -gdwarf-
  2, -gdwarf-3 or -gdwarf-4.
* Vectorizer improvements:

  o The straight-line code vectorizer now considers the whole function when
    vectorizing and can handle opportunities crossing CFG merges and backedges.

* A series of conditional expressions that compare the same variable can be
  transformed into a switch statement if each of them contains a comparison
  expression. Example:

            int IsHTMLWhitespace(int aChar) {
              return aChar == 0x0009 || aChar == 0x000A ||
                     aChar == 0x000C || aChar == 0x000D ||
                     aChar == 0x0020;
            }

  This statement can be transformed into a switch statement and then expanded
  into a bit-test.
* New command-line options:

  o -fbit-tests, enabled by default, can be used to enable or disable switch
    expansion using bit-tests.

* Inter-procedural optimization improvements:

  o A new IPA-modref pass was added to track side effects of function calls and
    improve precision of points-to-analysis. The pass can be controlled by the
    -fipa-modref option.
  o The identical code folding pass (controlled by -fipa-icf) was significantly
    improved to increase the number of unified functions and to reduce compile-
    time memory use.
  o IPA-CP (Interprocedural constant propagation) heuristics improved its
    estimation of potential usefulness of known loop bounds and strides by
    taking the estimated frequency of these loops into account.

* Link-time optimization improvements:

  o The LTO bytecode format was optimized for smaller object files and faster
    streaming.
  o Memory allocation of the linking stage was improved to reduce peak memory
    use.

* Profile driven optimization improvements:

  o Using -fprofile-values, was improved by tracking more target values for
    e.g. indirect calls.
  o GCOV data file format outputs smaller files by representing zero counters
    in a more compact way.



New Languages and Language specific improvements


* GCC 11 adds support for non-rectangular loop nests in OpenMP constructs and
  the allocator routines of OpenMP_5.0, including initial allocate clause
  support in C/C++. The OMP_TARGET_OFFLOAD environment variable and the active-
  levels routines are now supported. For C/C++, the declare variant and map
  support has been extended. For Fortran, OpenMP 4.5 is now fully supported and
  OpenMP 5.0 support has been extended, including the following features which
  were before only available in C and C++: order(concurrent), device_type,
  memorder-clauses for flush, lastprivate with conditional modifier, atomic
  construct and reduction clause extensions of OpenMP 5.0, if clause with simd
  and cancel modifiers, target data without map clause, and limited support for
  the requires construct.
* Version 2.6 of the OpenACC specification continues to be maintained and
  improved in the C, C++ and Fortran compilers. See the implementation_status
  section on the OpenACC wiki page and the run-time_library_documentation for
  further information.



C family


* New attributes:

  o The no_stack_protector attribute has been added to mark functions which
    should not be instrumented with stack protection (-fstack-protector).
  o The existing malloc attribute has been extended so that it can be used to
    identify allocator/deallocator API pairs. A pair of new -Wmismatched-
    dealloc and -Wmismatched-new-delete warnings will complain about mismatched
    calls, and -Wfree-nonheap-object about deallocation calls with pointers not
    obtained from allocation functions. Additionally, the static analyzer will
    use these attributes when checking for leaks, double-frees, use-after-
    frees, and similar issues.

* New warnings:

  o -Wmismatched-dealloc, enabled by default, warns about calls to deallocation
    functions with pointers returned from mismatched allocation functions.
  o -Wsizeof-array-div, enabled by -Wall, warns about divisions of two sizeof
    operators when the first one is applied to an array and the divisor does
    not equal the size of the array element.
  o -Wstringop-overread, enabled by default, warns about calls to string
    functions reading past the end of the arrays passed to them as arguments.
    In prior GCC releases most instances of his warning are diagnosed by -
    Wstringop-overflow.
  o -Wtsan, enabled by default, warns about unsupported features in
    ThreadSanitizer (currently std::atomic_thread_fence).

* Enhancements to existing warnings:

  o -Wfree-nonheap-object detects many more instances of calls to deallocation
    functions with pointers not returned from a dynamic memory allocation
    function.
  o -Wmaybe-uninitialized diagnoses passing pointers or references to
    uninitialized memory to functions taking const-qualified arguments.
  o -Wuninitialized detects reads from uninitialized dynamically allocated
    memory.

* For ELF targets that support the GNU or FreeBSD OSABIs, the used attribute
  will now save the symbol declaration it is applied to from linker garbage
  collection.
  To support this behavior, used symbols that have not been placed in specific
  sections (e.g. with the section attribute, or the -f{function,data}-sections
  options) will be placed in new, unique sections.
  This functionality requires Binutils version 2.36 or later.


C


* Several new features from the upcoming C2X revision of the ISO C standard are
  supported with -std=c2x and -std=gnu2x. Some of these features are also
  supported as extensions when compiling for older language versions. In
  addition to the features listed, some features previously supported as
  extensions and now added to the C standard are enabled by default in C2X mode
  and not diagnosed with -std=c2x -Wpedantic.

  o The BOOL_MAX and BOOL_WIDTH macros are provided in <limits.h>.
  o As in C++, function definitions no longer need to give names for unused
    function parameters.
  o The expansions of the true and false macros in <stdbool.h> have changed so
    that they have type bool.
  o The [[nodiscard]] standard attribute is now supported.
  o The __has_c_attribute preprocessor operator is now supported.
  o Macros INFINITY, NAN, FLT_SNAN, DBL_SNAN, LDBL_SNAN, DEC_INFINITY, DEC_NAN,
    and corresponding signaling NaN macros for _FloatN, _FloatNx and _DecimalN
    types, are provided in <float.h>. There are also corresponding built-in
    functions __builtin_nansdN for decimal signaling NaNs.
  o Macros FLT_IS_IEC_60559, DBL_IS_IEC_60559 and LDBL_IS_IEC_60559 are
    provided in <float.h>.
  o The feature test macro __STDC_WANT_IEC_60559_EXT__ is supported by
    <float.h>.
  o Labels may appear before declarations and at the end of a compound
    statement.

* New warnings:

  o -Warray-parameter, enabled by -Wall, warns about redeclarations of
    functions with ordinary array arguments declared using inconsistent forms.
    The warning also enables the detection of the likely out of bounds accesses
    in calls to such functions with smaller arrays.
  o -Wvla-parameter, enabled by -Wall, warns redeclarations of functions with
    variable length array arguments declared using inconsistent forms or with
    mismatched bounds. The warning also enables the detection of the likely out
    of bounds accesses in calls to such functions with smaller arrays.



C++


* The default mode has been changed to -std=gnu++17.
* Several C++20 features have been implemented:

  o the compiler now supports consteval virtual functions
  o P2082R1, Fixing CTAD for aggregates
  o P0593R6, Pseudo-destructors end object lifetimes
  o P1907R1, Inconsistencies with non-type template parameters (complete
    implementation)
  o P1975R0, Fixing the wording of parenthesized aggregate-initialization
  o P1009R2, Array size deduction in new-expressions
  o P1099R5, using enum
  o Modules, Requires -fmodules-ts and some aspects are incomplete. Refer to
    C++_20_Status

* The C++ front end has experimental support for some of the upcoming C++23
  draft features with the -std=c++23, -std=gnu++23, -std=c++2b or -std=gnu++2b
  flags, including

  o P0330R8, Literal Suffix for (signed) size_t.

  For a full list of new features, see the_C++_status_page.
* Several C++ Defect Reports have been resolved, e.g.:

  o DR 625, Use of auto as a template-argument
  o DR 1512, Pointer comparison vs qualification conversions
  o DR 1722, Should lambda to function pointer conversion function be noexcept?
  o DR 1914, Duplicate standard attributes
  o DR 2032, Default template-arguments of variable templates
  o DR 2289, Uniqueness of decomposition declaration names
  o DR 2237, Can a template-id name a constructor?
  o DR 2303, Partial ordering and recursive variadic inheritance
  o DR 2369, Ordering between constraints and substitution
  o DR 2450, braced-init-list as a template-argument

* G++ now performs better access checking in templates (PR41437).
* reinterpret_casts in constexpr evaluation are now checked more completely
  (PR95307).
* The diagnostic for static_assert has been improved: the compiler now shows
  the expression including its template arguments (if there were any), and can
  point to the failing clause if the condition comprised of any logical AND
  operators (PR97518).
* New warnings:

  o -Wctad-maybe-unsupported, disabled by default, warns about performing class
    template argument deduction on a type with no deduction guides.
  o -Wrange-loop-construct, enabled by -Wall, warns when a range-based for-loop
    is creating unnecessary and expensive copies.
  o -Wdeprecated-enum-enum-conversion, enabled by default in C++20, warns about
    deprecated arithmetic conversions on operands of enumeration types, as
    outlined in [depr.arith.conv.enum].
  o -Wdeprecated-enum-float-conversion, enabled by default in C++20, warns
    about deprecated arithmetic conversions on operands where one is of
    enumeration type and the other is of a floating-point type, as outlined in
    [depr.arith.conv.enum].
  o -Wmismatched-new-delete, enabled by -Wall, warns about calls to C++
    operator delete with pointers returned from mismatched forms of operator
    new or from other mismatched allocation functions.
  o -Wvexing-parse, enabled by default, warns about the most vexing parse rule:
    the cases when a declaration looks like a variable definition, but the C++
    language requires it to be interpreted as a function declaration.

* Enhancements to existing warnings:

  o -Wnonnull considers the implicit this argument of every C++ nonstatic
    member function to have been implicitly declared with attribute nonnull and
    triggers warnings for calls where the pointer is null.



Runtime Library (libstdc++)


* Improved C++17 support, including:

  o std::from_chars and std::to_chars for floating-point types.

* Improved experimental C++20 support, including:

  o Calendar additions to <chrono>. Thanks to Cassio Neri for optimizations.
  o std::bit_cast
  o std::source_location
  o Atomic wait and notify operations.
  o <barrier>, <latch>, and <semaphore>
  o <syncstream>
  o Efficient access to basic_stringbuf's buffer.

* Experimental C++23 support, including:

  o contains member functions for strings, thanks to Paul Fee.
  o std::to_underlying, std::is_scoped_enum

* Experimental support for Data-Parallel Types (simd) from the Parallelism 2
  TS, thanks to Matthias Kretz.
* Faster std::uniform_int_distribution, thanks to Daniel Lemire.


Fortran


* Added DEPRECATED to !GCC$'s attributes directive.



libgccjit


* libgccjit was marked as merely "Alpha" quality when originally_added_in_GCC
  5. Given that we have maintained API_and_ABI_compatibility since then and it
  is in use by various projects, we have removed that caveat.
* libgccjit can now be built for MinGW
* The libgccjit API gained 10 new entry points:

  o gcc_jit_global_set_initializer
  o 9 entrypoints for directly_embedding_asm_statements_into_a_compile,
    analogous to inline asm in the C front end



New Targets and Target Specific Improvements


AArch64 & arm


* A number of new CPUs are supported through arguments to the -mcpu and -mtune
  options in both the arm and aarch64 backends (GCC identifiers in
  parentheses):

  o Arm Cortex-A78 (cortex-a78).
  o Arm Cortex-A78AE (cortex-a78ae).
  o Arm Cortex-A78C (cortex-a78c).
  o Arm Cortex-X1 (cortex-x1).
  o Arm Neoverse V1 (neoverse-v1).
  o Arm Neoverse N2 (neoverse-n2).

* GCC can now auto-vectorize operations performing addition, subtraction,
  multiplication and the accumulate/subtract variants on complex numbers,
  taking advantage of the Advanced SIMD instructions in the Armv8.3-a (AArch64/
  AArch32), SVE (AArch64), SVE2 (AArch64) and MVE (AArch32 M-profile)
  instruction sets.


AArch64


* In addition to the above, the following AArch64-only CPUs are now supported:

  o Fujitsu A64FX (a64fx).
  o Arm Cortex-R82 (cortex-r82).

* The AArch64 Armv8-R architecture is now supported through the -march=armv8-
  r option.
* Mitigation against the Straight-line_Speculation_vulnerability is supported
  with the -mharden-sls= option. Please refer to the documentation for usage
  instructions.
* The availability of Advanced SIMD intrinsics available through the arm_neon.h
  header is improved and GCC 11 supports the full set of intrinsics defined by
  ACLE Q3 2020.


AMD Radeon (GCN)


* Initial support for gfx908 GPUs has been added.


arm


* Initial auto-vectorization is now available when targeting the MVE
  instruction set.
* GCC can now make use of the Low Overhead Branch instruction in Armv8.1-M to
  optimize loop counters and checks.
* The -mcpu=cortex-m55 option now supports the extensions +nomve and +nomve.fp
  to control generation of MVE and MVE floating-point instructions.



IA-32/x86-64


* New ISA extension support for Intel TSXLDTRK was added to GCC. TSXLDTRK
  intrinsics are available via the -mtsxldtrk compiler switch.
* New ISA extension support for Intel SERIALIZE was added to GCC. SERIALIZE
  intrinsics are available via the -mserialize compiler switch.
* New ISA extension support for Intel HRESET was added to GCC. HRESET
  intrinsics are available via the -mhreset compiler switch.
* New ISA extension support for Intel UINTR was added to GCC. UINTR intrinsics
  are available via the -muintr compiler switch.
* New ISA extension support for Intel KEYLOCKER was added to GCC. KEYLOCKER
  intrinsics are available via the -mkeylocker compiler switch.
* New ISA extension support for Intel AMX-TILE, AMX-INT8, AMX-BF16 was added to
  GCC. AMX-TILE, AMX-INT8, AMX-BF16 intrinsics are available via the -mamx-
  tile, -mamx-int8, -mamx-bf16 compiler switches.
* New ISA extension support for Intel AVX-VNNI was added to GCC. AVX-VNNI
  intrinsics are available via the -mavxvnni compiler switch.
* GCC now supports the Intel CPU named Sapphire Rapids through -
  march=sapphirerapids. The switch enables the MOVDIRI, MOVDIR64B,
  AVX512VP2INTERSECT, ENQCMD, CLDEMOTE, SERIALIZE, PTWRITE, WAITPKG, TSXLDTRK,
  AMT-TILE, AMX-INT8, AMX-BF16, and AVX-VNNI ISA extensions.
* GCC now supports the Intel CPU named Alderlake through -march=alderlake. The
  switch enables the CLDEMOTE, PTWRITE, WAITPKG, SERIALIZE, KEYLOCKER, AVX-
  VNNI, and HRESET ISA extensions.
* GCC now supports the Intel CPU named Rocketlake through -march=rocketlake.
  Rocket Lake is based on Icelake client and minus SGX.
* GCC now supports AMD CPUs based on the znver3 core via -march=znver3.
* GCC now supports micro-architecture levels defined in the x86-64 psABI via -
  march=x86-64-v2, -march=x86-64-v3 and -march=x86-64-v4.



Nios II


* The options -mcustom-insn=N no longer produce compiler warnings if the custom
  instruction is not generated due to missing optimization options such as -
  fno-math-errno, -ffinite-math-only, or -funsafe-math-optimizations. These
  warnings were not consistently emitted for all custom instructions.
* The -mcustom-fpu-cfg=fph2 option has been added to enable the custom
  instructions supported by the Nios II Floating Point Hardware 2 Component.


NVPTX


* The -misa default has changed from sm_30 to sm_35.
* The -m32 compiler switch has been removed.
* The -msoft-stack-reserve-local format has been fixed. Previously, it accepted
  -msoft-stack-reserve-local<n>. It now accepts -msoft-stack-reserve-local=<n>.



S/390, System z, IBM Z Systems


* The behavior when compiling with -fexcess-precision=standard (e.g., implied
  by -std=c99) on s390(x) targets can now be controlled at configure time with
  the flag --enable-s390-excess-float-precision. When enabled, GCC will
  maintain previous behavior and evaluate float expressions in double
  precision, which aligns with the definition of float_t as double. With the
  flag disabled, GCC will always evaluate float expressions in single
  precision. In native builds and cross compiles that have target libc headers,
  GCC will by default match the definition of float_t in the installed glibc.


RISC-V


* Support address sanitizer for RISC-V.
* Support big-endian for RISC-V, thanks to Marcus Comstedt.
* Implement new style of architecture extension test macros, each architecture
  extension has corresponding feature test macro, which could use to test its
  existence and version information.
* Legacy architecture extension test macro like __riscv_atomic, are deprecated,
  but it will still supported for at least 2 release cycles.
* Support IFUNC for riscv*-*-linux*.
* Add new option -misa-spec=* to control ISA spec version, default is 2.2, this
  option could control the default version of each extensions.
* Introduce --with-multilib-generator to configure time option, this option
  could flexible config multi-lib settings, syntax is same as RISC-V's
  multilib-generator.
* Extend the sytax for multilib-generator, support expansion operator * to
  reduce the complexity of complicated multi-lib re-use rule.
* Support -mcpu=* option, the behavior is aligned to RISC-V clang/LLVM, it will
  set pipeline model and architecture extension, like -mtune=* plus -march=*.
* Support for TLS stack protector canary access, thanks to Cooper Qu.
* Support __builtin_thread_pointer for RISC-V.
* Introduce shorten_memrefs optimization, which could reduce the code size for
  memory access, thanks to Craig Blackmore.



Operating Systems


AIX


* GCC for AIX can be built as a 64 bit application and the runtime is built as
  FAT libraries containing both 32 bit and 64 bit objects.
* Support AIX Vector Extended ABI with -mabi=vec-extabi.
* Thread-Local uninitiated data placed in local common section.
* Use thread-safe access in ctype.
* Link with libc128.a when long-double-128 enabled.



Improvements to Static Analyzer


* The implementation of how program state is tracked within -fanalyzer has been
  completely rewritten for GCC 11, fixing numerous bugs, and allowing for the
  analyzer to scale to larger C source files.
* The analysis of allocations and deallocations has been generalized beyond
  malloc and free.

  o As preliminary work towards eventually supporting C++, the malloc/free
    checking will also check new/delete and new[]/delete[]. However, C++ is not
    yet properly supported by -fanalyzer (for example, exception-handling is
    unimplemented).
  o As noted above, the existing malloc attribute has been extended so that it
    can be used to identify allocator/deallocator API pairs. The analyzer will
    use these attributes when checking for leaks, double-frees, use-after-
    frees, and similar issues.
  o A new -Wanalyzer-mismatching-deallocation warning has been added, covering
    such mismatches as using scalar delete rather vector delete[].

* The analyzer has gained warnings -Wanalyzer-shift-count-negative, -Wanalyzer-
  shift-count-overflow, -Wanalyzer-write-to-const, and -Wanalyzer-write-to-
  string-literal, all enabled by default when -fanalyzer is enabled.
* The analyzer can now be extended by GCC plugins, allowing for domain-specific
  path-sensitive warnings. An example of using a GCC_plugin_to_check_for
  misuses_of_CPython's_global_interpreter_lock can be seen in the test suite
* The analyzer has gained new debugging options -fdump-analyzer-json and -fno-
  analyzer-feasibility.



Other significant improvements


* GCC has gained a new environment variable GCC_EXTRA_DIAGNOSTIC_OUTPUT which
  can be used by IDEs to request machine-readable fix-it hints without needing
  to adjust build flags.


GCC 11.1

This is the list_of_problem_reports_(PRs) from GCC's bug tracking system that
are known to be fixed in the 11.1 release. This list might not be complete
(that is, it is possible that some PRs that have been fixed are not listed
here).