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From: Matthew Flatt <mflatt@racket-lang.org>
Date: Thu, 3 Sep 2020 05:56:20 -0600
Subject: make arm32 return-address encoding montonic w.r.t. offset
Forwarded: https://github.com/cisco/ChezScheme/pull/532
---
s/arm32.ss | 14 ++++++++++++--
1 file changed, 12 insertions(+), 2 deletions(-)
diff --git a/s/arm32.ss b/s/arm32.ss
index 9edb2ff..11b09e5 100644
--- a/s/arm32.ss
+++ b/s/arm32.ss
@@ -1679,6 +1679,16 @@
(bitwise-arithmetic-shift-left (logand n #xffffff) 8)
(bitwise-arithmetic-shift-right n 24)))))))))
+ ;; restrict funky12 so that an code offset n will not fit
+ ;; if a smaller offset wouldn't fit, which prevents bouncing
+ ;; in the loop that computes label offsets
+ (define code-offset-funky12
+ (lambda (n)
+ (safe-assert (and (fixnum? n) (fx= 0 (fxand n 3))))
+ (and (fixnum? n)
+ (#%$fxu< n #x400)
+ (funky12 n))))
+
(define shift-count?
(lambda (imm)
; can also allow 0 for lsl and 32 (represented as 0) for lsr, asr
@@ -2144,11 +2154,11 @@
(lambda (offset)
(let ([disp (fx- next-addr (fx- offset incr-offset) 4)])
(cond
- [(funky12 disp)
+ [(code-offset-funky12 disp)
(Trivit (dest)
; aka adr, encoding A1
(emit addi #f dest `(reg . ,%pc) disp '()))]
- [(funky12 (- disp))
+ [(code-offset-funky12 (- disp))
(Trivit (dest)
; aka adr, encoding A2
(emit subi #f dest `(reg . ,%pc) (- disp) '()))]
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