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[InstCombine] fix potential miscompile in select value equivalence 

As shown in the example based on:
https://llvm.org/PR49832
...and the existing test, we can't substitute
a vector value because the equality compare
replacement that we are attempting requires
that the comparison is true for the entire
value. Vector select can be partly true/false.
This commit is contained in:
Sanjay Patel 2021-04-05 12:14:49 -04:00
parent c0b0da4684
commit c590a9880d
2 changed files with 11 additions and 5 deletions
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5
llvm/lib/Transforms/InstCombine/InstCombineSelect.cpp
View File

@ -1124,7 +1124,10 @@ static Instruction *canonicalizeAbsNabs(SelectInst &Sel, ICmpInst &Cmp,
/// TODO: Wrapping flags could be preserved in some cases with better analysis.
Instruction *InstCombinerImpl::foldSelectValueEquivalence(SelectInst &Sel,
ICmpInst &Cmp) {
if (!Cmp.isEquality())
// Value equivalence substitution requires an all-or-nothing replacement.
// It does not make sense for a vector compare where each lane is chosen
// independently.
if (!Cmp.isEquality() || Cmp.getType()->isVectorTy())
return nullptr;
// Canonicalize the pattern to ICMP_EQ by swapping the select operands.

11
llvm/test/Transforms/InstCombine/select-binop-cmp.ll
View File

@ -551,12 +551,12 @@ define i32 @select_xor_icmp_bad_6(i32 %x, i32 %y, i32 %z) {
ret i32 %C
}
; FIXME: Value equivalence substitution is all-or-nothing, so needs a scalar compare.
; Value equivalence substitution is all-or-nothing, so needs a scalar compare.
define <2 x i8> @select_xor_icmp_vec_bad(<2 x i8> %x, <2 x i8> %y, <2 x i8> %z) {
; CHECK-LABEL: @select_xor_icmp_vec_bad(
; CHECK-NEXT: [[A:%.*]] = icmp eq <2 x i8> [[X:%.*]], <i8 5, i8 3>
; CHECK-NEXT: [[B:%.*]] = xor <2 x i8> [[Z:%.*]], <i8 5, i8 3>
; CHECK-NEXT: [[B:%.*]] = xor <2 x i8> [[X]], [[Z:%.*]]
; CHECK-NEXT: [[C:%.*]] = select <2 x i1> [[A]], <2 x i8> [[B]], <2 x i8> [[Y:%.*]]
; CHECK-NEXT: ret <2 x i8> [[C]]
;
@ -566,11 +566,14 @@ define <2 x i8> @select_xor_icmp_vec_bad(<2 x i8> %x, <2 x i8> %y, <2 x i8> %z)
ret <2 x i8> %C
}
; FIXME: Value equivalence substitution is all-or-nothing, so needs a scalar compare.
; Value equivalence substitution is all-or-nothing, so needs a scalar compare.
define <2 x i32> @vec_select_no_equivalence(<2 x i32> %x) {
; CHECK-LABEL: @vec_select_no_equivalence(
; CHECK-NEXT: ret <2 x i32> [[X:%.*]]
; CHECK-NEXT: [[X10:%.*]] = shufflevector <2 x i32> [[X:%.*]], <2 x i32> undef, <2 x i32> <i32 1, i32 0>
; CHECK-NEXT: [[COND:%.*]] = icmp eq <2 x i32> [[X]], zeroinitializer
; CHECK-NEXT: [[S:%.*]] = select <2 x i1> [[COND]], <2 x i32> [[X10]], <2 x i32> [[X]]
; CHECK-NEXT: ret <2 x i32> [[S]]
;
%x10 = shufflevector <2 x i32> %x, <2 x i32> undef, <2 x i32> <i32 1, i32 0>
%cond = icmp eq <2 x i32> %x, zeroinitializer