https://bugs.llvm.org/show_bug.cgi?id=46055
Bug ID: 46055
Summary: Invalid optimization: two different array indices are
considered equal
Product: clang
Version: 10.0
Hardware: PC
OS: Linux
Status: NEW
Severity: normal
Priority: P
Component: C
Assignee: unassignedclangb...@nondot.org
Reporter: br...@clisp.org
CC: blitzrak...@gmail.com, dgre...@apple.com,
erik.pilking...@gmail.com, llvm-bugs@lists.llvm.org,
richard-l...@metafoo.co.uk
Created attachment 23527
--> https://bugs.llvm.org/attachment.cgi?id=23527&action=edit
Test case
The attached program, foo.c, ought to exit with code 3 if calloc() fails, and
with code 2 if calloc() succeeds.
Without optimization, it's as expected:
$ clang -Wall foo.c
$ ./a.out; echo $?
3
With optimization, it's wrong:
$ clang -Wall -O2 foo.c
$ ./a.out; echo $?
0
Here's the output of the clang optimizer:
$ clang -Wall -O2 -S foo.c && cat foo.s
.text
.file "foo.c"
.globl main # -- Begin function main
.p2align 4, 0x90
.type main,@function
main: # @main
.cfi_startproc
# %bb.0:
xorl %eax, %eax
retq
.Lfunc_end0:
.size main, .Lfunc_end0-main
.cfi_endproc
# -- End function
.ident "clang version 10.0.0 "
.section ".note.GNU-stack","",@progbits
.addrsig
As you can see, clang must have evaluated the condition (s[n - 1].c[0]) to
true. But since the memory of s was freshly allocated and zero-filled and the
index n-1 is different from 0, this condition ought to have evaluated to false.
Probably the bug is related to the fact that (n-1) * sizeof (S8) is a multiple
of 2^64.
If clang is assuming a flat address space (of size 2^64), it may indeed
simplify (n-1) * sizeof (S8) to zero, but then it must not assume that calloc()
will return a non-NULL pointer.
If clang is NOT assuming a flat address space, it must not simplify (n-1) *
sizeof (S8) to zero.
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