[llvm] [ValueTracking] Try to infer range of select from true and false values. (PR #68256)

[via llvm-commits]

llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-llvm-transforms

<details>
<summary>Changes</summary>

When computing range of `select` instruction, first compute the union of ranges of "True" and "False" operands of the `select` instruction.

---
Full diff: https://github.com/llvm/llvm-project/pull/68256.diff


3 Files Affected:

- (modified) llvm/lib/Analysis/ValueTracking.cpp (+7-2) 
- (added) llvm/test/Analysis/ValueTracking/constant-range-select.ll (+15) 
- (modified) llvm/test/Transforms/InstCombine/binop-select.ll (+2-2) 


``````````diff
diff --git a/llvm/lib/Analysis/ValueTracking.cpp b/llvm/lib/Analysis/ValueTracking.cpp
index 0736ef65b306519..4078376124b1750 100644
--- a/llvm/lib/Analysis/ValueTracking.cpp
+++ b/llvm/lib/Analysis/ValueTracking.cpp
@@ -8854,11 +8854,16 @@ ConstantRange llvm::computeConstantRange(const Value *V, bool ForSigned,
   } else if (auto *II = dyn_cast<IntrinsicInst>(V))
     CR = getRangeForIntrinsic(*II);
   else if (auto *SI = dyn_cast<SelectInst>(V)) {
+    ConstantRange CRTrue = computeConstantRange(
+        SI->getTrueValue(), ForSigned, UseInstrInfo, AC, CtxI, DT, Depth + 1);
+    ConstantRange CRFalse = computeConstantRange(
+        SI->getFalseValue(), ForSigned, UseInstrInfo, AC, CtxI, DT, Depth + 1);
+    CR = CRTrue.unionWith(CRFalse);
+    // TODO: Return ConstantRange.
     APInt Lower = APInt(BitWidth, 0);
     APInt Upper = APInt(BitWidth, 0);
-    // TODO: Return ConstantRange.
     setLimitsForSelectPattern(*SI, Lower, Upper, IIQ);
-    CR = ConstantRange::getNonEmpty(Lower, Upper);
+    CR = CR.intersectWith(ConstantRange::getNonEmpty(Lower, Upper));
   } else if (isa<FPToUIInst>(V) || isa<FPToSIInst>(V)) {
     APInt Lower = APInt(BitWidth, 0);
     APInt Upper = APInt(BitWidth, 0);
diff --git a/llvm/test/Analysis/ValueTracking/constant-range-select.ll b/llvm/test/Analysis/ValueTracking/constant-range-select.ll
new file mode 100644
index 000000000000000..353fa7e1cf5dd33
--- /dev/null
+++ b/llvm/test/Analysis/ValueTracking/constant-range-select.ll
@@ -0,0 +1,15 @@
+; RUN: opt < %s -aa-pipeline=basic-aa -passes=aa-eval -print-all-alias-modref-info -disable-output 2>&1 | FileCheck %s
+
+ at a = dso_local local_unnamed_addr global [10 x i32] zeroinitializer, align 4
+
+; CHECK-LABEL: Function: select_in_gep
+; CHECK: NoAlias: i32* %arrayidx, i32* getelementptr inbounds ([10 x i32], ptr @a, i64 0, i64 3)
+define i32 @select_in_gep(i1 %c)  {
+entry:
+  %cond = select i1 %c, i64 2, i64 1
+  %0 = load i32, ptr getelementptr inbounds ([10 x i32], ptr @a, i64 0, i64 3), align 4
+  %arrayidx = getelementptr inbounds [10 x i32], ptr @a, i64 0, i64 %cond
+  store i32 %0, ptr %arrayidx, align 4
+  %1 = load i32, ptr getelementptr inbounds ([10 x i32], ptr @a, i64 0, i64 3), align 4
+  ret i32 %1
+}
diff --git a/llvm/test/Transforms/InstCombine/binop-select.ll b/llvm/test/Transforms/InstCombine/binop-select.ll
index a59e19897f061d1..6cd4132eadd77b9 100644
--- a/llvm/test/Transforms/InstCombine/binop-select.ll
+++ b/llvm/test/Transforms/InstCombine/binop-select.ll
@@ -324,12 +324,12 @@ define i32 @sub_sel_op1_use(i1 %b) {
 ; CHECK-LABEL: @sub_sel_op1_use(
 ; CHECK-NEXT:    [[S:%.*]] = select i1 [[B:%.*]], i32 42, i32 41
 ; CHECK-NEXT:    call void @use(i32 [[S]])
-; CHECK-NEXT:    [[R:%.*]] = sub nsw i32 42, [[S]]
+; CHECK-NEXT:    [[R:%.*]] = sub nuw nsw i32 42, [[S]]
 ; CHECK-NEXT:    ret i32 [[R]]
 ;
   %s = select i1 %b, i32 42, i32 41
   call void @use(i32 %s)
-  %r = sub nsw i32 42, %s
+  %r = sub nuw nsw i32 42, %s
   ret i32 %r
 }
 

``````````

</details>


https://github.com/llvm/llvm-project/pull/68256