[ValueLattice] Support constant vectors in mergeIn()

[nikic]

This is a followup to vector support in LVI/CVP/SCCP. In mergeIn(), if one of the operands is a vector of integer constant, we should try to convert it into a constant range, in case that allows performing a range union to something better than overdefined.

[llvmbot]

@llvm/pr-subscribers-llvm-transforms

@llvm/pr-subscribers-llvm-analysis

Author: Nikita Popov (nikic)

Changes

This is a followup to vector support in LVI/CVP/SCCP. In mergeIn(), if one of the operands is a vector of integer constant, we should try to convert it into a constant range, in case that allows performing a range union to something better than overdefined.

---

Full diff: https://github.com/llvm/llvm-project/pull/99466.diff

3 Files Affected:

• (modified) llvm/include/llvm/Analysis/ValueLattice.h (+22-12)
• (modified) llvm/test/Transforms/CorrelatedValuePropagation/vectors.ll (+2-2)
• (modified) llvm/test/Transforms/SCCP/phis.ll (+2-2)

diff --git a/llvm/include/llvm/Analysis/ValueLattice.h b/llvm/include/llvm/Analysis/ValueLattice.h
index b81eb5f60ab7e..fa56d838a3859 100644
--- a/llvm/include/llvm/Analysis/ValueLattice.h
+++ b/llvm/include/llvm/Analysis/ValueLattice.h
@@ -281,18 +281,21 @@ class ValueLatticeElement {
     return std::nullopt;
   }
 
-  ConstantRange asConstantRange(Type *Ty, bool UndefAllowed = false) const {
-    assert(Ty->isIntOrIntVectorTy() && "Must be integer type");
+  ConstantRange asConstantRange(unsigned BW, bool UndefAllowed = false) const {
     if (isConstantRange(UndefAllowed))
       return getConstantRange();
     if (isConstant())
       return getConstant()->toConstantRange();
-    unsigned BW = Ty->getScalarSizeInBits();
     if (isUnknown())
       return ConstantRange::getEmpty(BW);
     return ConstantRange::getFull(BW);
   }
 
+  ConstantRange asConstantRange(Type *Ty, bool UndefAllowed = false) const {
+    assert(Ty->isIntOrIntVectorTy() && "Must be integer type");
+    return asConstantRange(Ty->getScalarSizeInBits(), UndefAllowed);
+  }
+
   bool markOverdefined() {
     if (isOverdefined())
       return false;
@@ -384,7 +387,9 @@ class ValueLatticeElement {
       return true;
     }
 
-    assert(isUnknown() || isUndef());
+    assert(isUnknown() || isUndef() || isConstant());
+    assert((!isConstant() || NewR.contains(getConstant()->toConstantRange())) &&
+           "Constant must be subset of new range");
 
     NumRangeExtensions = 0;
     Tag = NewTag;
@@ -426,6 +431,16 @@ class ValueLatticeElement {
         return false;
       if (RHS.isUndef())
         return false;
+      // If the constant is a vector of integers, try to treat it as a range.
+      if (getConstant()->getType()->isVectorTy() &&
+          getConstant()->getType()->getScalarType()->isIntegerTy()) {
+        ConstantRange L = getConstant()->toConstantRange();
+        ConstantRange NewR = L.unionWith(
+            RHS.asConstantRange(L.getBitWidth(), /*UndefAllowed=*/true));
+        return markConstantRange(
+            std::move(NewR),
+            Opts.setMayIncludeUndef(RHS.isConstantRangeIncludingUndef()));
+      }
       markOverdefined();
       return true;
     }
@@ -444,14 +459,9 @@ class ValueLatticeElement {
       return OldTag != Tag;
     }
 
-    if (!RHS.isConstantRange()) {
-      // We can get here if we've encountered a constantexpr of integer type
-      // and merge it with a constantrange.
-      markOverdefined();
-      return true;
-    }
-
-    ConstantRange NewR = getConstantRange().unionWith(RHS.getConstantRange());
+    const ConstantRange &L = getConstantRange();
+    ConstantRange NewR = L.unionWith(
+        RHS.asConstantRange(L.getBitWidth(), /*UndefAllowed=*/true));
     return markConstantRange(
         std::move(NewR),
         Opts.setMayIncludeUndef(RHS.isConstantRangeIncludingUndef()));
diff --git a/llvm/test/Transforms/CorrelatedValuePropagation/vectors.ll b/llvm/test/Transforms/CorrelatedValuePropagation/vectors.ll
index 43e680cd25cdb..6254b54d42554 100644
--- a/llvm/test/Transforms/CorrelatedValuePropagation/vectors.ll
+++ b/llvm/test/Transforms/CorrelatedValuePropagation/vectors.ll
@@ -286,7 +286,7 @@ define <2 x i16> @phi_merge1(i1 %c, <2 x i8> %a) {
 ; CHECK-NEXT:    br label %[[JOIN]]
 ; CHECK:       [[JOIN]]:
 ; CHECK-NEXT:    [[PHI:%.*]] = phi <2 x i16> [ [[ZEXT]], %[[ENTRY]] ], [ <i16 1, i16 2>, %[[IF]] ]
-; CHECK-NEXT:    [[ADD:%.*]] = add <2 x i16> [[PHI]], <i16 2, i16 3>
+; CHECK-NEXT:    [[ADD:%.*]] = add nuw nsw <2 x i16> [[PHI]], <i16 2, i16 3>
 ; CHECK-NEXT:    ret <2 x i16> [[ADD]]
 ;
 entry:
@@ -312,7 +312,7 @@ define <2 x i16> @phi_merge2(i1 %c, <2 x i8> %a) {
 ; CHECK-NEXT:    br label %[[JOIN]]
 ; CHECK:       [[JOIN]]:
 ; CHECK-NEXT:    [[PHI:%.*]] = phi <2 x i16> [ <i16 1, i16 2>, %[[ENTRY]] ], [ [[ZEXT]], %[[IF]] ]
-; CHECK-NEXT:    [[ADD:%.*]] = add <2 x i16> [[PHI]], <i16 2, i16 3>
+; CHECK-NEXT:    [[ADD:%.*]] = add nuw nsw <2 x i16> [[PHI]], <i16 2, i16 3>
 ; CHECK-NEXT:    ret <2 x i16> [[ADD]]
 ;
 entry:
diff --git a/llvm/test/Transforms/SCCP/phis.ll b/llvm/test/Transforms/SCCP/phis.ll
index 83daae0a7c0c8..9264a6eaefb85 100644
--- a/llvm/test/Transforms/SCCP/phis.ll
+++ b/llvm/test/Transforms/SCCP/phis.ll
@@ -109,7 +109,7 @@ define <2 x i16> @phi_vector_merge1(i1 %c, <2 x i8> %a) {
 ; CHECK-NEXT:    br label %[[JOIN]]
 ; CHECK:       [[JOIN]]:
 ; CHECK-NEXT:    [[PHI:%.*]] = phi <2 x i16> [ [[ZEXT]], %[[ENTRY]] ], [ <i16 1, i16 2>, %[[IF]] ]
-; CHECK-NEXT:    [[ADD:%.*]] = add <2 x i16> [[PHI]], <i16 2, i16 3>
+; CHECK-NEXT:    [[ADD:%.*]] = add nuw nsw <2 x i16> [[PHI]], <i16 2, i16 3>
 ; CHECK-NEXT:    ret <2 x i16> [[ADD]]
 ;
 entry:
@@ -135,7 +135,7 @@ define <2 x i16> @phi_vector_merge2(i1 %c, <2 x i8> %a) {
 ; CHECK-NEXT:    br label %[[JOIN]]
 ; CHECK:       [[JOIN]]:
 ; CHECK-NEXT:    [[PHI:%.*]] = phi <2 x i16> [ <i16 1, i16 2>, %[[ENTRY]] ], [ [[ZEXT]], %[[IF]] ]
-; CHECK-NEXT:    [[ADD:%.*]] = add <2 x i16> [[PHI]], <i16 2, i16 3>
+; CHECK-NEXT:    [[ADD:%.*]] = add nuw nsw <2 x i16> [[PHI]], <i16 2, i16 3>
 ; CHECK-NEXT:    ret <2 x i16> [[ADD]]
 ;
 entry:

[dtcxzyw]

Do you know the reason that we weren't allowed to convert a constant into a constant range?

[nikic]

For plain integers, constants get converted into constant range on construction. For vectors this doesn't happen as it's a lossy operation. So I think there was just never a need for it before.

[dtcxzyw]

LGTM.