https://llvm.org/bugs/show_bug.cgi?id=31096
Bug ID: 31096
Summary: Multi-level pointers not disambiguated, even with
strict aliasing
Product: libraries
Version: trunk
Hardware: PC
OS: All
Status: NEW
Severity: normal
Priority: P
Component: Global Analyses
Assignee: unassignedb...@nondot.org
Reporter: dber...@dberlin.org
CC: llvm-bugs@lists.llvm.org
Classification: Unclassified
The following simple example:
int *p;
int foo(int argc, char **argv)
{
int result;
*p = 2;
if (argc)
*p = 2;
result = *p;
return result;
}
Does not get optimized by GVN (or NewGVN).
It's trivial friend:
int foo(int argc, char **argv, int *p)
{
int result;
*p = 2;
if (argc)
*p = 2;
result = *p;
return result;
}
Does get optimized by NewGVN with a patch i have.
The difference is that the global object turns into an i32 ** that we do a load
from, so the llvm IR looks like this:
@p = common global i32* null, align 8
; Function Attrs: norecurse nounwind ssp uwtable
define i32 @foo(i32, i8** nocapture readnone) #0 {
%3 = load i32*, i32** @p, align 8, !tbaa !2
store i32 2, i32* %3, align 4, !tbaa !6
%4 = icmp eq i32 %0, 0
br i1 %4, label %7, label %5
; <label>:5 ; preds = %2
%6 = load i32*, i32** @p, align 8, !tbaa !2
store i32 2, i32* %6, align 4, !tbaa !6
br label %7
; <label>:7 ; preds = %2, %5
%8 = load i32*, i32** @p, align 8, !tbaa !2
%9 = load i32, i32* %8, align 4, !tbaa !6
ret i32 %9
}
!0 = !{i32 1, !"PIC Level", i32 2}
!1 = !{!"Apple LLVM version 8.0.0 (clang-800.0.42.1)"}
!2 = !{!3, !3, i64 0}
!3 = !{!"any pointer", !4, i64 0}
!4 = !{!"omnipotent char", !5, i64 0}
!5 = !{!"Simple C/C++ TBAA"}
!6 = !{!7, !7, i64 0}
!7 = !{!"int", !4, i64 0}
Note that in this two level version, we believe the stores to i32* can affect
an i32**.
At the "llvm level" this is correct (IE there are no real types, aso we can't
say anything) , however, with strict aliasing, and the original code, it's not
possible.
In fact, at least for basic types, with strict aliasing, it's not possible for
a pointer of level n to affect a pointer of level n+1 (IE a store to an int *
can't ever alias an int **, only the reverse) , but we haven't taught LLVM
that.
(I suspect, btw, the above can be reproduced with multi-level pointers in
general)
Not sure where we should fix this, suggestions welcome.
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