https://llvm.org/bugs/show_bug.cgi?id=29057
Bug ID: 29057
Summary: GVN PRE not respecting the "inbounds" keyword of GEP
Product: libraries
Version: trunk
Hardware: PC
OS: Linux
Status: NEW
Severity: normal
Priority: P
Component: Scalar Optimizations
Assignee: [email protected]
Reporter: [email protected]
CC: [email protected]
Classification: Unclassified
Related with: https://llvm.org/bugs/show_bug.cgi?id=28562
As stated, GVN ignores the "inbounds" keyword of GEP during numbering.
And it actually performs PRE on GEP regardless of "inbounds" keyword.
Below is the actual code transformation. (Using 'opt -gvn')
source:
================================================================
define i32* @foo(i32* %a, i1 %cond) {
entry:
br i1 %cond, label %brA, label %brB
brA:
%xA = getelementptr inbounds i32, i32* %a, i32 10
br label %out
brB:
%xB = getelementptr inbounds i32, i32* %a, i32 10
br label %out
out:
%x = getelementptr i32, i32* %a, i32 10
ret i32* %x
}
================================================================
target:
================================================================
define i32* @foo(i32* %a, i1 %cond) {
entry:
br i1 %cond, label %brA, label %brB
brA: ; preds = %entry
%xA = getelementptr inbounds i32, i32* %a, i32 10
br label %out
brB: ; preds = %entry
%xB = getelementptr inbounds i32, i32* %a, i32 10
br label %out
out: ; preds = %brB, %brA
%x.pre-phi = phi i32* [ %xB, %brB ], [ %xA, %brA ]
ret i32* %x.pre-phi
}
================================================================
This may be problematic because GEP with "inbounds" keyword may produce poison
value while normal one does not, and poison value may produce UB. More behavior
in target code is problematic.
One possible solution may to use "patchAndReplaceAllUsesWith" instead of naive
"replaceAllUsesWith" in "performScalarPRE".
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