[ValueTracking] Analyze Select in isKnownNonEqual.

[mgudim]

Basic way to recursively analyze select in isKnownNonEqual: select %c, %t, %f is non-equal to %x if %t is non-equal to %x and %f is non-equal to %x.

[llvmbot]

@llvm/pr-subscribers-llvm-analysis

Changes

Basic way to recursively analyze select in isKnownNonEqual: select %c, %t, %f is non-equal to %x if %t is non-equal to %x and %f is non-equal to %x.

---

Full diff: https://github.com/llvm/llvm-project/pull/68427.diff

2 Files Affected:

• (modified) llvm/lib/Analysis/ValueTracking.cpp (+20)
• (added) llvm/test/Analysis/BasicAA/non-equal-select.ll (+33)

diff --git a/llvm/lib/Analysis/ValueTracking.cpp b/llvm/lib/Analysis/ValueTracking.cpp
index 3af5a6d9a167de4..1c284e8d85ca417 100644
--- a/llvm/lib/Analysis/ValueTracking.cpp
+++ b/llvm/lib/Analysis/ValueTracking.cpp
@@ -3112,6 +3112,23 @@ static bool isNonEqualPHIs(const PHINode *PN1, const PHINode *PN2,
   return true;
 }
 
+static bool isNonEqualSELECT(const Value *V1, const Value *V2, unsigned Depth,
+                             const SimplifyQuery &Q) {
+  const SelectInst *SI1 = dyn_cast<SelectInst>(V1);
+  if (!SI1)
+    return false;
+
+  if (const SelectInst *SI2 = dyn_cast<SelectInst>(V2)) {
+    if (SI1->getCondition() == SI2->getCondition())
+      return isKnownNonEqual(SI1->getTrueValue(), SI2->getTrueValue(),
+                             Depth + 1, Q) &&
+             isKnownNonEqual(SI1->getFalseValue(), SI2->getFalseValue(),
+                             Depth + 1, Q);
+  }
+  return isKnownNonEqual(SI1->getTrueValue(), V2, Depth + 1, Q) &&
+         isKnownNonEqual(SI1->getFalseValue(), V2, Depth + 1, Q);
+}
+
 /// Return true if it is known that V1 != V2.
 static bool isKnownNonEqual(const Value *V1, const Value *V2, unsigned Depth,
                             const SimplifyQuery &Q) {
@@ -3142,6 +3159,9 @@ static bool isKnownNonEqual(const Value *V1, const Value *V2, unsigned Depth,
     };
   }
 
+  if (isNonEqualSELECT(V1, V2, Depth, Q) || isNonEqualSELECT(V2, V1, Depth, Q))
+    return true;
+
   if (isAddOfNonZero(V1, V2, Depth, Q) || isAddOfNonZero(V2, V1, Depth, Q))
     return true;
 
diff --git a/llvm/test/Analysis/BasicAA/non-equal-select.ll b/llvm/test/Analysis/BasicAA/non-equal-select.ll
new file mode 100644
index 000000000000000..1af37245c6d85a6
--- /dev/null
+++ b/llvm/test/Analysis/BasicAA/non-equal-select.ll
@@ -0,0 +1,33 @@
+; RUN: opt < %s -aa-pipeline=basic-aa -passes=aa-eval -print-all-alias-modref-info -disable-output 2>&1 | FileCheck %s
+@G = global [10 x i32] zeroinitializer, align 4
+
+; Function Attrs: mustprogress nofree norecurse nosync nounwind willreturn memory(readwrite, argmem: none, inaccessiblemem: none) uwtable
+define void @select_in_gep1(i1 %c, i64 noundef %x) {
+entry:
+; CHECK: Function: select_in_gep1
+; CHECK: NoAlias: i32* %arrayidx1, i32* %arrayidx2
+  %add1_ = add nsw i64 %x, 1
+  %add2_ = add nsw i64 %x, 2
+  %select_ = select i1 %c, i64 %add1_, i64 %add2_
+  %arrayidx1 = getelementptr inbounds [10 x i32], ptr @G, i64 0, i64 %select_
+  store i32 42, ptr %arrayidx1, align 4
+  %arrayidx2 = getelementptr inbounds [10 x i32], ptr @G, i64 0, i64 %x
+  store i32 43, ptr %arrayidx2, align 4
+  ret void
+}
+
+define void @select_in_gep2(i1 %c, i64 noundef %x) {
+entry:
+  ; TODO: should be "NoAlias" here as well.
+; CHECK: Function: select_in_gep2
+; CHECK: MayAlias:     i32* %arrayidx1, i32* %arrayidx2
+  %add1_ = add nsw i64 %x, 1
+  %add2_ = add nsw i64 %x, 2
+  %add3_ = add nsw i64 %x, 3
+  %select_ = select i1 %c, i64 %add1_, i64 %add2_
+  %arrayidx1 = getelementptr inbounds [10 x i32], ptr @G, i64 0, i64 %select_
+  store i32 42, ptr %arrayidx1, align 4
+  %arrayidx2 = getelementptr inbounds [10 x i32], ptr @G, i64 0, i64 %add3_
+  store i32 43, ptr %arrayidx2, align 4
+  ret void
+}

[github-actions]

✅ With the latest revision this PR passed the C/C++ code formatter.

[mgudim]

@nikic @goldsteinn I added a new commit moving the isNonEqualSelect to the end of isKnownNonEqual.

This is necessary so that we can analyze things like
isKnownNonEqual(X, Y) where X = select ... and Y = shl X, 2 for
example. In other words, we first should try to do the analysis without
"decomposing" select. If that doesn't succeed then we try isNonEqualSelect.

[nikic]

@nikic @goldsteinn I added a new commit moving the isNonEqualSelect to the end of isKnownNonEqual.

This is necessary so that we can analyze things like isKnownNonEqual(X, Y) where X = select ... and Y = shl X, 2 for example. In other words, we first should try to do the analysis without "decomposing" select. If that doesn't succeed then we try isNonEqualSelect.

I don't understand what you mean here. If isNonEqualSelect() returns false, then all the following code still gets executed, no? It shouldn't matter (for behavior) in which order these calls are executed.

[mgudim]

I don't understand what you mean here. If isNonEqualSelect() returns false, then all the following code still gets executed, no? It shouldn't matter (for behavior) in which order these calls are executed.

Right, I just thought it's better to do cheaper checks first.

[mgudim]

@nikic @goldsteinn ping

[nikic]

LGTM