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[BasicAA] Handle known non-zero variable index 

BasicAA currently handles cases like Scale*V0 + (-Scale)*V1 where
V0 != V1, but does not handle the simpler case of Scale*V with
V != 0. Add it based on an isKnownNonZero() call.

I'm not passing a context instruction for now, because the existing
approach of always using GEP1 for context could result in symmetry
issues.

Differential Revision: https://reviews.llvm.org/D93162
This commit is contained in:
Nikita Popov 2020-12-10 22:29:33 +01:00
parent 05d1729232
commit bb939ebfd7
3 changed files with 11 additions and 9 deletions
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8
llvm/lib/Analysis/BasicAliasAnalysis.cpp
View File

@ -1288,8 +1288,12 @@ AliasResult BasicAAResult::aliasGEP(
if (V1Size.hasValue() && V2Size.hasValue()) {
// Try to determine whether abs(VarIndex) > 0.
Optional<APInt> MinAbsVarIndex;
// TODO: Could handle single non-zero index as well.
if (DecompGEP1.VarIndices.size() == 2) {
if (DecompGEP1.VarIndices.size() == 1) {
// VarIndex = Scale*V. If V != 0 then abs(VarIndex) >= abs(Scale).
const VariableGEPIndex &Var = DecompGEP1.VarIndices[0];
if (isKnownNonZero(Var.V, DL))
MinAbsVarIndex = Var.Scale.abs();
} else if (DecompGEP1.VarIndices.size() == 2) {
// VarIndex = Scale*V0 + (-Scale)*V1.
// If V0 != V1 then abs(VarIndex) >= abs(Scale).
// Check that VisitedPhiBBs is empty, to avoid reasoning about

4
llvm/test/Analysis/BasicAA/bug.23626.ll
View File

@ -4,9 +4,9 @@ target triple = "x86_64-apple-darwin13.4.0"
; CHECK-LABEL: compute1
; CHECK: MayAlias: i32* %arrayidx8, i32* %out
; CHECK: MayAlias: i32* %arrayidx11, i32* %out
; CHECK: NoAlias: i32* %arrayidx11, i32* %out
; CHECK: MayAlias: i32* %arrayidx11, i32* %arrayidx8
; CHECK: MayAlias: i32* %arrayidx14, i32* %out
; CHECK: NoAlias: i32* %arrayidx14, i32* %out
; CHECK: MayAlias: i32* %arrayidx14, i32* %arrayidx8
; CHECK: MayAlias: i32* %arrayidx11, i32* %arrayidx14
define void @compute1(i32 %num.0.lcssa, i32* %out) {

8
llvm/test/Analysis/BasicAA/sequential-gep.ll
View File

@ -111,10 +111,9 @@ define void @add_non_zero_with_offset(i32* %p, i32 %addend, i32* %q) {
}
; CHECK-LABEL: non_zero_index_simple
; CHECK: MayAlias: i32* %gep, i32* %p
; CHECK: MayAlias: i16* %gep.16, i32* %p
; CHECK: NoAlias: i32* %gep, i32* %p
; CHECK: NoAlias: i16* %gep.16, i32* %p
; CHECK: MayAlias: i32* %p, i64* %gep.64
; TODO: First two could be NoAlias.
define void @non_zero_index_simple(i32* %p, i32* %q) {
%knownnonzero = load i32, i32* %q, !range !0
%gep = getelementptr i32, i32* %p, i32 %knownnonzero
@ -125,8 +124,7 @@ define void @non_zero_index_simple(i32* %p, i32* %q) {
; CHECK-LABEL: non_zero_index_with_offset
; CHECK: MayAlias: i32* %gep, i32* %p
; CHECK: MayAlias: i16* %gep.16, i32* %p
; TODO: Last could be NoAlias.
; CHECK: NoAlias: i16* %gep.16, i32* %p
define void @non_zero_index_with_offset(i32* %p, i32* %q) {
%knownnonzero = load i32, i32* %q, !range !0
%p.8 = bitcast i32* %p to i8*