[DemandedBits] Fix computation of demanded bits for ICmps

The computation of ICmp demanded bits is independent of the individual operand being evaluated. We simply return a mask consisting of the minimum leading zeroes of both operands.

We were incorrectly passing "I" to ComputeKnownBits - this should be "UserI->getOperand(0)". In cases where we were evaluating the 1th operand, we were taking the minimum leading zeroes of it and itself.

This should fix PR26266.

llvm-svn: 258690

diff --git a/llvm/lib/Analysis/DemandedBits.cpp b/llvm/lib/Analysis/DemandedBits.cpp
index 912c5ce..143d0b7 100644 (file)
--- a/llvm/lib/Analysis/DemandedBits.cpp
+++ b/llvm/lib/Analysis/DemandedBits.cpp
@@ -244,7 +244,7 @@ void DemandedBits::determineLiveOperandBits(
     break;
   case Instruction::ICmp:
     // Count the number of leading zeroes in each operand.
-    ComputeKnownBits(BitWidth, I, UserI->getOperand(1));
+    ComputeKnownBits(BitWidth, UserI->getOperand(0), UserI->getOperand(1));
     auto NumLeadingZeroes = std::min(KnownZero.countLeadingOnes(),
                                      KnownZero2.countLeadingOnes());
     AB = ~APInt::getHighBitsSet(BitWidth, NumLeadingZeroes);

diff --git a/llvm/test/Analysis/DemandedBits/basic.ll b/llvm/test/Analysis/DemandedBits/basic.ll
index 487e522..9973edf 100644 (file)
--- a/llvm/test/Analysis/DemandedBits/basic.ll
+++ b/llvm/test/Analysis/DemandedBits/basic.ll
@@ -24,11 +24,20 @@ define i1 @test_icmp1(i32 %a, i32 %b) {
 
 ; CHECK-LABEL: 'test_icmp2'
 ; CHECK-DAG: DemandedBits: 0x1 for   %3 = icmp eq i32 %1, %2
-; CHECK-DAG: DemandedBits: 0xFF for   %1 = and i32 %a, 255
-; CHECK-DAG: DemandedBits: 0xF for   %2 = ashr i32 %1, 4
+; CHECK-DAG: DemandedBits: 0xFFF for   %1 = and i32 %a, 255
+; CHECK-DAG: DemandedBits: 0xFF for   %2 = ashr i32 %1, 4
 define i1 @test_icmp2(i32 %a, i32 %b) {
   %1 = and i32 %a, 255
   %2 = ashr i32 %1, 4
   %3 = icmp eq i32 %1, %2
   ret i1 %3
 }
+
+; CHECK-LABEL: 'test_icmp3'
+; CHECK-DAG: DemandedBits: 0xFFFFFFFF for   %1 = and i32 %a, 255
+; CHECK-DAG: DemandedBits: 0x1 for   %2 = icmp eq i32 -1, %1
+define i1 @test_icmp3(i32 %a) {
+  %1 = and i32 %a, 255
+  %2 = icmp eq i32 -1, %1
+  ret i1 %2
+}