[llvm] 154c8a0 - [InstCombine] Use KnownBits::ashr()

[Nikita Popov via llvm-commits]

Author: Nikita Popov
Date: 2024-07-01T14:52:06+02:00
New Revision: 154c8a02ed952fe3fa2e074fce52a07c4d1efab2

URL: https://github.com/llvm/llvm-project/commit/154c8a02ed952fe3fa2e074fce52a07c4d1efab2
DIFF: https://github.com/llvm/llvm-project/commit/154c8a02ed952fe3fa2e074fce52a07c4d1efab2.diff

LOG: [InstCombine] Use KnownBits::ashr()

This fixes a consistency violation under -instcombine-verify-known-bits.

Added: 
    

Modified: 
    llvm/lib/Transforms/InstCombine/InstCombineSimplifyDemanded.cpp
    llvm/test/Transforms/InstCombine/pr80597.ll

Removed: 
    


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diff  --git a/llvm/lib/Transforms/InstCombine/InstCombineSimplifyDemanded.cpp b/llvm/lib/Transforms/InstCombine/InstCombineSimplifyDemanded.cpp
index 02003150d85da..ca6034e09b73b 100644
--- a/llvm/lib/Transforms/InstCombine/InstCombineSimplifyDemanded.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstCombineSimplifyDemanded.cpp
@@ -812,15 +812,15 @@ Value *InstCombinerImpl::SimplifyDemandedUseBits(Value *V, APInt DemandedMask,
         return I;
       }
 
-      // Compute the new bits that are at the top now plus sign bits.
-      APInt HighBits(APInt::getHighBitsSet(
-          BitWidth, std::min(SignBits + ShiftAmt - 1, BitWidth)));
-      Known.Zero.lshrInPlace(ShiftAmt);
-      Known.One.lshrInPlace(ShiftAmt);
+      Known = KnownBits::ashr(
+          Known, KnownBits::makeConstant(APInt(BitWidth, ShiftAmt)),
+          ShiftAmt != 0, I->isExact());
 
       // If the input sign bit is known to be zero, or if none of the top bits
       // are demanded, turn this into an unsigned shift right.
       assert(BitWidth > ShiftAmt && "Shift amount not saturated?");
+      APInt HighBits(APInt::getHighBitsSet(
+          BitWidth, std::min(SignBits + ShiftAmt - 1, BitWidth)));
       if (Known.Zero[BitWidth-ShiftAmt-1] ||
           !DemandedMask.intersects(HighBits)) {
         BinaryOperator *LShr = BinaryOperator::CreateLShr(I->getOperand(0),
@@ -828,11 +828,6 @@ Value *InstCombinerImpl::SimplifyDemandedUseBits(Value *V, APInt DemandedMask,
         LShr->setIsExact(cast<BinaryOperator>(I)->isExact());
         LShr->takeName(I);
         return InsertNewInstWith(LShr, I->getIterator());
-      } else if (Known.One[BitWidth-ShiftAmt-1]) { // New bits are known one.
-        Known.One |= HighBits;
-        // SignBits may be out-of-sync with Known.countMinSignBits(). Mask out
-        // high bits of Known.Zero to avoid conflicts.
-        Known.Zero &= ~HighBits;
       }
     } else {
       llvm::computeKnownBits(I, Known, Depth, Q);

diff  --git a/llvm/test/Transforms/InstCombine/pr80597.ll b/llvm/test/Transforms/InstCombine/pr80597.ll
index 5feae4a06c45c..148da056486f9 100644
--- a/llvm/test/Transforms/InstCombine/pr80597.ll
+++ b/llvm/test/Transforms/InstCombine/pr80597.ll
@@ -11,7 +11,7 @@ define i64 @pr80597(i1 %cond) {
 ; CHECK-NEXT:    br i1 [[CMP]], label [[IF_THEN:%.*]], label [[IF_ELSE:%.*]]
 ; CHECK:       if.else:
 ; CHECK-NEXT:    [[SEXT2:%.*]] = ashr exact i64 [[ADD]], 1
-; CHECK-NEXT:    [[ASHR:%.*]] = or i64 [[SEXT2]], 4418419761487020032
+; CHECK-NEXT:    [[ASHR:%.*]] = or disjoint i64 [[SEXT2]], 4418419761487020032
 ; CHECK-NEXT:    ret i64 [[ASHR]]
 ; CHECK:       if.then:
 ; CHECK-NEXT:    ret i64 0