[BasicAA] Account for wrapping when using abs(Scale*V0 + (-Scale)*V1) >= abs(Scale) #137755
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… >= abs(Scale) Similar to 1b7ef6a, add a check to only set MinAbsVarIndex if abs(Scale*V0) and abs((-Scale)*V1) won't wrap. In the absence of IsNSW, try to use the bitwidths of the original V and Scale to rule out wrapping
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@llvm/pr-subscribers-llvm-analysis Author: Craig Topper (topperc) ChangesSimilar to 1b7ef6a, add a check to Full diff: https://github.com/llvm/llvm-project/pull/137755.diff 2 Files Affected:
diff --git a/llvm/lib/Analysis/BasicAliasAnalysis.cpp b/llvm/lib/Analysis/BasicAliasAnalysis.cpp
index cbec13c7808be..03977551afadd 100644
--- a/llvm/lib/Analysis/BasicAliasAnalysis.cpp
+++ b/llvm/lib/Analysis/BasicAliasAnalysis.cpp
@@ -1301,6 +1301,23 @@ AliasResult BasicAAResult::aliasGEP(
if (Range1.intersectWith(Range2).isEmptySet())
return AliasResult::NoAlias;
+ // Check if abs(V*Scale) >= abs(Scale) holds in the presence of
+ // potentially wrapping math.
+ auto MultiplyByScaleNoWrap = [](const VariableGEPIndex &Var) {
+ if (Var.IsNSW)
+ return true;
+
+ int ValOrigBW = Var.Val.V->getType()->getPrimitiveSizeInBits();
+ // If Scale is small enough so that abs(V*Scale) >= abs(Scale) holds.
+ // The max value of abs(V) is 2^ValOrigBW - 1. Multiplying with a
+ // constant smaller than 2^(bitwidth(Val) - ValOrigBW) won't wrap.
+ int MaxScaleValueBW = Var.Val.getBitWidth() - ValOrigBW;
+ if (MaxScaleValueBW <= 0)
+ return false;
+ return Var.Scale.ule(
+ APInt::getMaxValue(MaxScaleValueBW).zext(Var.Scale.getBitWidth()));
+ };
+
// Try to determine the range of values for VarIndex such that
// VarIndex <= -MinAbsVarIndex || MinAbsVarIndex <= VarIndex.
std::optional<APInt> MinAbsVarIndex;
@@ -1309,22 +1326,6 @@ AliasResult BasicAAResult::aliasGEP(
const VariableGEPIndex &Var = DecompGEP1.VarIndices[0];
if (Var.Val.TruncBits == 0 &&
isKnownNonZero(Var.Val.V, SimplifyQuery(DL, DT, &AC, Var.CxtI))) {
- // Check if abs(V*Scale) >= abs(Scale) holds in the presence of
- // potentially wrapping math.
- auto MultiplyByScaleNoWrap = [](const VariableGEPIndex &Var) {
- if (Var.IsNSW)
- return true;
-
- int ValOrigBW = Var.Val.V->getType()->getPrimitiveSizeInBits();
- // If Scale is small enough so that abs(V*Scale) >= abs(Scale) holds.
- // The max value of abs(V) is 2^ValOrigBW - 1. Multiplying with a
- // constant smaller than 2^(bitwidth(Val) - ValOrigBW) won't wrap.
- int MaxScaleValueBW = Var.Val.getBitWidth() - ValOrigBW;
- if (MaxScaleValueBW <= 0)
- return false;
- return Var.Scale.ule(
- APInt::getMaxValue(MaxScaleValueBW).zext(Var.Scale.getBitWidth()));
- };
// Refine MinAbsVarIndex, if abs(Scale*V) >= abs(Scale) holds in the
// presence of potentially wrapping math.
if (MultiplyByScaleNoWrap(Var)) {
@@ -1345,7 +1346,8 @@ AliasResult BasicAAResult::aliasGEP(
SimplifyQuery(DL, DT, &AC, /*CxtI=*/Var0.CxtI
? Var0.CxtI
: Var1.CxtI)))
- MinAbsVarIndex = Var0.Scale.abs();
+ if (MultiplyByScaleNoWrap(Var0) && MultiplyByScaleNoWrap(Var1))
+ MinAbsVarIndex = Var0.Scale.abs();
}
if (MinAbsVarIndex) {
diff --git a/llvm/test/Analysis/BasicAA/gep-modulo.ll b/llvm/test/Analysis/BasicAA/gep-modulo.ll
index 159aaf94039d3..9ca69b2d7bbdd 100644
--- a/llvm/test/Analysis/BasicAA/gep-modulo.ll
+++ b/llvm/test/Analysis/BasicAA/gep-modulo.ll
@@ -393,3 +393,17 @@ entry:
}
declare void @llvm.assume(i1)
+
+define i64 @mul_may_overflow_var_nonzero_minabsvarindex_two_index(i64 %arg, ptr %arg1) {
+; CHECK-LABEL: Function: mul_may_overflow_var_nonzero_minabsvarindex_two_index
+; CHECK-LABEL: MayAlias: i64* %getelementptr, i64* %getelementptr2
+bb:
+ %xor = xor i64 %arg, -9223372036854775808
+ %getelementptr = getelementptr i64, ptr %arg1, i64 %xor
+ %load = load i64, ptr %getelementptr, align 8
+ %getelementptr2 = getelementptr i64, ptr %arg1, i64 %arg
+ store i64 1, ptr %getelementptr2, align 8
+ %load3 = load i64, ptr %getelementptr, align 8
+ %mul = mul i64 %load, %load3
+ ret i64 %mul
+}
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nikic
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Apr 29, 2025
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LGTM
Approximate proof: https://alive2.llvm.org/ce/z/hTla9G This is not quite correct for the non-IsNegated case (src2), but I think we'd only hit that in cases where NSW is dropped earlier already (
llvm-project/llvm/lib/Analysis/BasicAliasAnalysis.cpp
Lines 1872 to 1874 in a9ece2d
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LLVM Buildbot has detected a new failure on builder Full details are available at: https://lab.llvm.org/buildbot/#/builders/88/builds/11076 Here is the relevant piece of the build log for the reference |
IanWood1
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May 6, 2025
… >= abs(Scale) (llvm#137755) Similar to 1b7ef6a, add a check to only set MinAbsVarIndex if abs(Scale*V0) and abs((-Scale)*V1) won't wrap. In the absence of IsNSW, try to use the bitwidths of the original V and Scale to rule out wrapping
GeorgeARM
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to GeorgeARM/llvm-project
that referenced
this pull request
May 7, 2025
… >= abs(Scale) (llvm#137755) Similar to 1b7ef6a, add a check to only set MinAbsVarIndex if abs(Scale*V0) and abs((-Scale)*V1) won't wrap. In the absence of IsNSW, try to use the bitwidths of the original V and Scale to rule out wrapping
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Similar to 1b7ef6a, add a check to only set MinAbsVarIndex if abs(Scale*V0) and abs((-Scale)*V1) won't wrap. In the absence of IsNSW, try to use the bitwidths of the original V and Scale to rule out wrapping