https://llvm.org/bugs/show_bug.cgi?id=27418
Bug ID: 27418
Summary: Different index types in GEPs -> non-aliasing
Product: libraries
Version: trunk
Hardware: PC
OS: Linux
Status: NEW
Severity: normal
Priority: P
Component: Global Analyses
Assignee: unassignedb...@nondot.org
Reporter: mikael.hol...@ericsson.com
CC: llvm-bugs@lists.llvm.org
Classification: Unclassified
Created attachment 16233
--> https://llvm.org/bugs/attachment.cgi?id=16233&action=edit
Reproducer ll file
It seems that opt thinks that the two pointers
%_tmp2 = getelementptr [3 x i16], [3 x i16]* %a, i16 0, i64 1
and
%_tmp4 = getelementptr [3 x i16], [3 x i16]* %a, i16 0, i16 1
does not alias? Is this intended or a bug?
Details below:
--------------
I found this when I ran opt on:
define i16 @f () {
%a = alloca [3 x i16]
; Write 98 at index 1 in the array.
; NB: using i64 as type of the index argument!
%_tmp2 = getelementptr [3 x i16], [3 x i16]* %a, i16 0, i64 1
store i16 98, i16* %_tmp2
; Read at index 1 in the array
; NB: using i16 as type of the index argument!
%_tmp4 = getelementptr [3 x i16], [3 x i16]* %a, i16 0, i16 1
%_tmp5 = load i16, i16* %_tmp4
; Check read value. If not 98, jump to bb1
%_tmp6 = icmp ne i16 %_tmp5, 98
br i1 %_tmp6, label %bb1, label %bb2
bb1:
; Error
ret i16 1;
bb2:
; Ok
ret i16 0;
}
opt -S -gvn foo.ll
and I got the result
; ModuleID = 'foo.ll'
define i16 @f() {
%a = alloca [3 x i16]
%_tmp2 = getelementptr [3 x i16], [3 x i16]* %a, i16 0, i64 1
store i16 98, i16* %_tmp2
%_tmp4 = getelementptr [3 x i16], [3 x i16]* %a, i16 0, i16 1
br i1 undef, label %bb1, label %bb2
bb1: ; preds = %0
ret i16 1
bb2: ; preds = %0
ret i16 0
}
Note the "undef" in the branch which GVN put there:
GVN iteration: 0
GVN removed: %_tmp5 = load i16, i16* %_tmp4
GVN removed: %_tmp6 = icmp ne i16 undef, 98
When digging into why GVN does this, I found this little piece of code in
GVN::AnalyzeLoadAvailability that triggers
// Loading the allocation -> undef.
if (isa<AllocaInst>(DepInst) || isMallocLikeFn(DepInst, TLI) ||
// Loading immediately after lifetime begin -> undef.
isLifetimeStart(DepInst)) {
Res = AvailableValue::get(UndefValue::get(LI->getType()));
return true;
}
And the DepInst here is
%a = alloca [3 x i16]
and not the store
store i16 98, i16* %_tmp2
Digging further it seems the reason for this is that the alias analysis
considers the two pointers used to store and load the value 98 are
non-aliasing, even if both the pointer and the indices has the same values. The
only difference is the type of the last value.
In e.g. BasicAliasAnalysis.cpp we have
bool BasicAAResult::isValueEqualInPotentialCycles(const Value *V,
const Value *V2) {
if (V != V2)
return false;
and in
aliasSameBasePointerGEPs:
// If the last (struct) indices are constants and are equal, the other
indices
// might be also be dynamically equal, so the GEPs can alias.
if (C1 && C2 && C1 == C2)
return MayAlias;
so here we consider two pointers to be completely different just because the
types of the last indices differ.
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