[llvm] 1b7ef6a - [BasicAA] Account for wrapping when using abs(VarIndex) >= abs(Scale).
Florian Hahn via llvm-commits
llvm-commits at lists.llvm.org
Fri Mar 18 07:41:22 PDT 2022
Author: Florian Hahn
Date: 2022-03-18T14:41:15Z
New Revision: 1b7ef6aac8a3cad245c0ed14fe21725e31261f73
URL: https://github.com/llvm/llvm-project/commit/1b7ef6aac8a3cad245c0ed14fe21725e31261f73
DIFF: https://github.com/llvm/llvm-project/commit/1b7ef6aac8a3cad245c0ed14fe21725e31261f73.diff
LOG: [BasicAA] Account for wrapping when using abs(VarIndex) >= abs(Scale).
The patch adds an extra check to only set MinAbsVarIndex if
abs(V * Scale) won't wrap. In the absence of IsNSW, try to use the
bitwidths of the original V and Scale to rule out wrapping.
Attempt to model https://alive2.llvm.org/ce/z/HE8ZKj
The code in the else if below probably needs the same treatment, but I
need to come up with a test first.
Reviewed By: asbirlea
Differential Revision: https://reviews.llvm.org/D121695
Added:
Modified:
llvm/lib/Analysis/BasicAliasAnalysis.cpp
llvm/test/Analysis/BasicAA/gep-modulo.ll
Removed:
################################################################################
diff --git a/llvm/lib/Analysis/BasicAliasAnalysis.cpp b/llvm/lib/Analysis/BasicAliasAnalysis.cpp
index 8c11cdf4834d6..e8d0c76730c95 100644
--- a/llvm/lib/Analysis/BasicAliasAnalysis.cpp
+++ b/llvm/lib/Analysis/BasicAliasAnalysis.cpp
@@ -1297,8 +1297,31 @@ AliasResult BasicAAResult::aliasGEP(
const VariableGEPIndex &Var = DecompGEP1.VarIndices[0];
if (Var.Val.TruncBits == 0 &&
isKnownNonZero(Var.Val.V, DL, 0, &AC, Var.CxtI, DT)) {
- // If V != 0 then abs(VarIndex) >= abs(Scale).
- MinAbsVarIndex = Var.Scale.abs();
+ // If V != 0, then abs(VarIndex) > 0.
+ MinAbsVarIndex = APInt(Var.Scale.getBitWidth(), 1);
+
+ // 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)) {
+ // If V != 0 then abs(VarIndex) >= abs(Scale).
+ MinAbsVarIndex = Var.Scale.abs();
+ }
}
} else if (DecompGEP1.VarIndices.size() == 2) {
// VarIndex = Scale*V0 + (-Scale)*V1.
diff --git a/llvm/test/Analysis/BasicAA/gep-modulo.ll b/llvm/test/Analysis/BasicAA/gep-modulo.ll
index 6e1779679d686..7e62a15aac47d 100644
--- a/llvm/test/Analysis/BasicAA/gep-modulo.ll
+++ b/llvm/test/Analysis/BasicAA/gep-modulo.ll
@@ -323,9 +323,9 @@ define void @may_overflow_mul_scale_neg([200 x [ 6 x i8]]* %ptr, i64 %idx.1,i64
; If %v == 10581764700698480926, %idx == 917, so %gep.917 and %gep.idx may alias.
define i8 @mul_may_overflow_var_nonzero_minabsvarindex_one_index([2000 x i8]* %arr, i8 %x, i64 %v) {
; CHECK-LABEL: Function: mul_may_overflow_var_nonzero_minabsvarindex_one_index: 4 pointers, 0 call sites
-; CHECK-NEXT: NoAlias: [2000 x i8]* %arr, i8* %gep.idx
+; CHECK-NEXT: MayAlias: [2000 x i8]* %arr, i8* %gep.idx
; CHECK-NEXT: PartialAlias (off 917): [2000 x i8]* %arr, i8* %gep.917
-; CHECK-NEXT: NoAlias: i8* %gep.917, i8* %gep.idx
+; CHECK-NEXT: MayAlias: i8* %gep.917, i8* %gep.idx
; CHECK-NEXT: MustAlias: [2000 x i8]* %arr, i8* %gep.0
; CHECK-NEXT: NoAlias: i8* %gep.0, i8* %gep.idx
; CHECK-NEXT: NoAlias: i8* %gep.0, i8* %gep.917
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