https://llvm.org/bugs/show_bug.cgi?id=25822
Bug ID: 25822
Summary: Bug in alias analysis (GlobalsAA)
Product: libraries
Version: trunk
Hardware: PC
OS: Linux
Status: NEW
Severity: normal
Priority: P
Component: Global Analyses
Assignee: unassignedb...@nondot.org
Reporter: fra...@codeplay.com
CC: llvm-bugs@lists.llvm.org
Classification: Unclassified
I believe I've found a bug in alias analysis (globals mod ref) leading to
incorrect instcombine optimization. Take the following C snippet:
static int i;
int __attribute__((noinline)) foo(int *ptr) {
i = 1;
*ptr = 10;
return i;
}
Compiled with clang to IR at -O0 we get:
@i = internal global i32 0, align 4
; Function Attrs: noinline nounwind uwtable
define i32 @foo(i32* %ptr) #0 {
entry:
%ptr.addr = alloca i32*, align 8
store i32* %ptr, i32** %ptr.addr, align 8
store i32 1, i32* @i, align 4
%0 = load i32*, i32** %ptr.addr, align 8
store i32 10, i32* %0, align 4
%1 = load i32, i32* @i, align 4
ret i32 %1
}
Then running opt on it as follows:
> opt -O1 -S -o - test.ll
@i = internal global i32 0, align 4
; Function Attrs: noinline norecurse nounwind uwtable
define i32 @foo(i32* nocapture %ptr) #0 {
entry:
store i32 1, i32* @i, align 4
store i32 10, i32* %ptr, align 4
ret i32 1
}
I believe this is an invalid optimization, as ptr could alias to &i, like in
this example:
int main() {
return foo(&i) != 10;
}
I tracked it down to GlobalsAA being turned on by default in r250157 (git
938c3d3)
So, doing:
> opt -O1 -S -enable-non-lto-gmr=false -o - test.ll
@i = internal global i32 0, align 4
; Function Attrs: noinline norecurse nounwind uwtable
define i32 @foo(i32* nocapture %ptr) #0 {
entry:
store i32 1, i32* @i, align 4
store i32 10, i32* %ptr, align 4
%0 = load i32, i32* @i, align 4
ret i32 %0
}
works as expected. I haven't gone further to work out what inside these
analyses is causing the problem.
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