[llvm] [ValueTracking] Take into account wether zero is poison when computing CR for `ct{t,l}z` (PR #122548)

[via llvm-commits]

llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-llvm-analysis

Author: None (goldsteinn)

<details>
<summary>Changes</summary>



---
Full diff: https://github.com/llvm/llvm-project/pull/122548.diff


2 Files Affected:

- (modified) llvm/lib/Analysis/ValueTracking.cpp (+11-4) 
- (modified) llvm/test/Transforms/InstSimplify/call.ll (+2-6) 


``````````diff
diff --git a/llvm/lib/Analysis/ValueTracking.cpp b/llvm/lib/Analysis/ValueTracking.cpp
index 4b246c013e96f1..15057901cd74c7 100644
--- a/llvm/lib/Analysis/ValueTracking.cpp
+++ b/llvm/lib/Analysis/ValueTracking.cpp
@@ -9898,13 +9898,20 @@ static void setLimitsForBinOp(const BinaryOperator &BO, APInt &Lower,
   }
 }
 
-static ConstantRange getRangeForIntrinsic(const IntrinsicInst &II) {
+static ConstantRange getRangeForIntrinsic(const IntrinsicInst &II,
+                                          bool UseInstrInfo) {
   unsigned Width = II.getType()->getScalarSizeInBits();
   const APInt *C;
   switch (II.getIntrinsicID()) {
-  case Intrinsic::ctpop:
   case Intrinsic::ctlz:
-  case Intrinsic::cttz:
+  case Intrinsic::cttz: {
+    APInt Upper = APInt(Width, Width);
+    if (!UseInstrInfo || !match(II.getArgOperand(1), m_One()))
+      Upper += 1;
+    // Maximum of set/clear bits is the bit width.
+    return ConstantRange::getNonEmpty(APInt::getZero(Width), Upper);
+  }
+  case Intrinsic::ctpop:
     // Maximum of set/clear bits is the bit width.
     return ConstantRange::getNonEmpty(APInt::getZero(Width),
                                       APInt(Width, Width) + 1);
@@ -10095,7 +10102,7 @@ ConstantRange llvm::computeConstantRange(const Value *V, bool ForSigned,
     setLimitsForBinOp(*BO, Lower, Upper, IIQ, ForSigned);
     CR = ConstantRange::getNonEmpty(Lower, Upper);
   } else if (auto *II = dyn_cast<IntrinsicInst>(V))
-    CR = getRangeForIntrinsic(*II);
+    CR = getRangeForIntrinsic(*II, UseInstrInfo);
   else if (auto *SI = dyn_cast<SelectInst>(V)) {
     ConstantRange CRTrue = computeConstantRange(
         SI->getTrueValue(), ForSigned, UseInstrInfo, AC, CtxI, DT, Depth + 1);
diff --git a/llvm/test/Transforms/InstSimplify/call.ll b/llvm/test/Transforms/InstSimplify/call.ll
index 67d5c4dbfb2e7d..910750adf9d427 100644
--- a/llvm/test/Transforms/InstSimplify/call.ll
+++ b/llvm/test/Transforms/InstSimplify/call.ll
@@ -1582,9 +1582,7 @@ define i1 @ctlz_i1_non_poison_eq_false(i1 %x) {
 
 define i1 @ctlz_i1_poison_eq_false(i1 %x) {
 ; CHECK-LABEL: @ctlz_i1_poison_eq_false(
-; CHECK-NEXT:    [[CT:%.*]] = call i1 @llvm.ctlz.i1(i1 [[X:%.*]], i1 true)
-; CHECK-NEXT:    [[CMP:%.*]] = icmp eq i1 [[CT]], false
-; CHECK-NEXT:    ret i1 [[CMP]]
+; CHECK-NEXT:    ret i1 true
 ;
   %ct = call i1 @llvm.ctlz.i1(i1 %x, i1 true)
   %cmp = icmp eq i1 %ct, false
@@ -1604,9 +1602,7 @@ define i1 @cttz_i1_non_poison_eq_false(i1 %x) {
 
 define i1 @cttz_i1_poison_eq_false(i1 %x) {
 ; CHECK-LABEL: @cttz_i1_poison_eq_false(
-; CHECK-NEXT:    [[CT:%.*]] = call i1 @llvm.cttz.i1(i1 [[X:%.*]], i1 true)
-; CHECK-NEXT:    [[CMP:%.*]] = icmp eq i1 [[CT]], false
-; CHECK-NEXT:    ret i1 [[CMP]]
+; CHECK-NEXT:    ret i1 true
 ;
   %ct = call i1 @llvm.cttz.i1(i1 %x, i1 true)
   %cmp = icmp eq i1 %ct, false

``````````

</details>


https://github.com/llvm/llvm-project/pull/122548