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commit 6a311a7a804831fea43cfb2f61322adcb407a1af
Author: Keno Fischer <keno@juliacomputing.com>
Date: Thu Jan 18 15:57:05 2018 -0500
[JumpThreading] Don't restrict cast-traversal to i1
Summary:
In D17663, JumpThreading learned to look trough simple cast instructions,
but only if the source of those cast instructions was a phi/cmp i1
(in an effort to limit compile time effects). I think this condition
is too restrictive. For switches with limited value range, InstCombine
will readily introduce an extra `trunc` instruction to a smaller
integer type (e.g. from i8 to i2), leaving us in the somewhat perverse
situation that jump-threading would work before running instcombine,
but not after. Since instcombine produces this pattern, I think we
need to consider it canonical and support it in JumpThreading.
In general, for limiting recursion, I think the existing restriction
to phi and cmp nodes should be sufficient to avoid looking through
unprofitable chains of instructions.
Reviewers: haicheng, gberry, bmakam, mcrosier
Subscribers: llvm-commits
Differential Revision: https://reviews.llvm.org/D42262
diff --git a/lib/Transforms/Scalar/JumpThreading.cpp b/lib/Transforms/Scalar/JumpThreading.cpp
index 95c4650..1155e18 100644
--- a/lib/Transforms/Scalar/JumpThreading.cpp
+++ b/lib/Transforms/Scalar/JumpThreading.cpp
@@ -647,11 +647,9 @@ bool JumpThreadingPass::ComputeValueKnownInPredecessors(
}
// Handle Cast instructions. Only see through Cast when the source operand is
- // PHI or Cmp and the source type is i1 to save the compilation time.
+ // PHI or Cmp to save the compilation time.
if (CastInst *CI = dyn_cast<CastInst>(I)) {
Value *Source = CI->getOperand(0);
- if (!Source->getType()->isIntegerTy(1))
- return false;
if (!isa<PHINode>(Source) && !isa<CmpInst>(Source))
return false;
ComputeValueKnownInPredecessors(Source, BB, Result, Preference, CxtI);
diff --git a/test/Transforms/JumpThreading/basic.ll b/test/Transforms/JumpThreading/basic.ll
index ce86cba..16e7549 100644
--- a/test/Transforms/JumpThreading/basic.ll
+++ b/test/Transforms/JumpThreading/basic.ll
@@ -547,6 +547,34 @@ l5:
; CHECK: }
}
+define i1 @trunc_switch(i1 %arg) {
+; CHECK-LABEL: @trunc_switch
+top:
+; CHECK: br i1 %arg, label %exitA, label %exitB
+ br i1 %arg, label %common, label %B
+
+B:
+ br label %common
+
+common:
+ %phi = phi i8 [ 2, %B ], [ 1, %top ]
+ %trunc = trunc i8 %phi to i2
+; CHECK-NOT: switch
+ switch i2 %trunc, label %unreach [
+ i2 1, label %exitA
+ i2 -2, label %exitB
+ ]
+
+unreach:
+ unreachable
+
+exitA:
+ ret i1 true
+
+exitB:
+ ret i1 false
+}
+
; CHECK-LABEL: define void @h_con(i32 %p) {
define void @h_con(i32 %p) {
%x = icmp ult i32 %p, 5
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