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[InstCombine] Do not fold 'and (sext (ashr X, Shift)), C' if Shift < 0 

The 'and (sext (ashr X, ShiftC)), C' --> 'lshr (sext X), ShiftC'
transformation would access out of bounds bits in APInt::getLowBitsSet
if the shift count was larger than X's bit width or if it was negative.

Fixes #56424
This commit is contained in:
Daniel Bertalan 2022-07-07 17:31:59 +02:00
parent 6656029a49
commit ef7aed3e11
2 changed files with 29 additions and 1 deletions
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3
llvm/lib/Transforms/InstCombine/InstCombineAndOrXor.cpp
View File

@ -1808,7 +1808,8 @@ Instruction *InstCombinerImpl::visitAnd(BinaryOperator &I) {
unsigned Width = Ty->getScalarSizeInBits();
const APInt *ShiftC;
if (match(Op0, m_OneUse(m_SExt(m_AShr(m_Value(X), m_APInt(ShiftC)))))) {
if (match(Op0, m_OneUse(m_SExt(m_AShr(m_Value(X), m_APInt(ShiftC))))) &&
ShiftC->ult(Width)) {
if (*C == APInt::getLowBitsSet(Width, Width - ShiftC->getZExtValue())) {
// We are clearing high bits that were potentially set by sext+ashr:
// and (sext (ashr X, ShiftC)), C --> lshr (sext X), ShiftC

27
llvm/test/Transforms/InstCombine/pr56424.ll Normal file
View File

@ -0,0 +1,27 @@
; NOTE: Assertions have been autogenerated by utils/update_test_checks.py
; RUN: opt < %s --passes=instcombine -S | FileCheck %s
; This would crash if we didn't check for a negative shift.
; https://github.com/llvm/llvm-project/issues/56424
define i64 @PR56424(i1 %cond, i32 %arg) {
; CHECK-LABEL: @PR56424(
; CHECK-NEXT: entry:
; CHECK-NEXT: br i1 [[COND:%.*]], label [[IF_THEN:%.*]], label [[IF_END:%.*]]
; CHECK: if.then:
; CHECK-NEXT: br label [[IF_END]]
; CHECK: if.end:
; CHECK-NEXT: ret i64 0
;
entry:
br i1 %cond, label %if.then, label %if.end
if.then:
%shr = ashr i32 %arg, -2
%sext = sext i32 %shr to i64
br label %if.end
if.end:
%val = phi i64 [ %sext, %if.then ], [ 0, %entry ]
%and = and i64 -81, %val
ret i64 %and
}