[ConstraintElim] Don't decompose values wider than 64 bits

[nikic]

Our coefficients are 64-bits, so adding/multiplying them can wrap in 64-bits even if there would be no wrapping the full bit width.

The alternative would be to check for overflows during all adds/muls in decomposition. I assume that we don't particularly care about handling wide integers here, so I've opted to bail out.

Fixes #68751.

[llvmbot]

@llvm/pr-subscribers-llvm-transforms

Author: Nikita Popov (nikic)

Changes

Our coefficients are 64-bits, so adding/multiplying them can wrap in 64-bits even if there would be no wrapping the full bit width.

The alternative would be to check for overflows during all adds/muls in decomposition. I assume that we don't particularly care about handling wide integers here, so I've opted to bail out.

Fixes #68751.

---

Full diff: https://github.com/llvm/llvm-project/pull/68803.diff

3 Files Affected:

• (modified) llvm/lib/Transforms/Scalar/ConstraintElimination.cpp (+13-3)
• (modified) llvm/test/Transforms/ConstraintElimination/large-constant-ints.ll (+5-3)
• (modified) llvm/test/Transforms/ConstraintElimination/shl.ll (+2-1)

diff --git a/llvm/lib/Transforms/Scalar/ConstraintElimination.cpp b/llvm/lib/Transforms/Scalar/ConstraintElimination.cpp
index 1eb7e481d43cd2b..37f720ec40f4e54 100644
--- a/llvm/lib/Transforms/Scalar/ConstraintElimination.cpp
+++ b/llvm/lib/Transforms/Scalar/ConstraintElimination.cpp
@@ -451,6 +451,19 @@ static Decomposition decompose(Value *V,
     return ResA;
   };
 
+  Type *Ty = V->getType()->getScalarType();
+  if (Ty->isPointerTy() && !IsSigned) {
+    if (auto *GEP = dyn_cast<GEPOperator>(V))
+      return decomposeGEP(*GEP, Preconditions, IsSigned, DL);
+    return V;
+  }
+
+  // Don't handle integers > 64 bit. Our coefficients are 64-bit large, so
+  // coefficient add/mul may wrap, while the operation in the full bit width
+  // would not.
+  if (!Ty->isIntegerTy() || Ty->getIntegerBitWidth() > 64)
+    return V;
+
   // Decompose \p V used with a signed predicate.
   if (IsSigned) {
     if (auto *CI = dyn_cast<ConstantInt>(V)) {
@@ -478,9 +491,6 @@ static Decomposition decompose(Value *V,
     return int64_t(CI->getZExtValue());
   }
 
-  if (auto *GEP = dyn_cast<GEPOperator>(V))
-    return decomposeGEP(*GEP, Preconditions, IsSigned, DL);
-
   Value *Op0;
   bool IsKnownNonNegative = false;
   if (match(V, m_ZExt(m_Value(Op0)))) {
diff --git a/llvm/test/Transforms/ConstraintElimination/large-constant-ints.ll b/llvm/test/Transforms/ConstraintElimination/large-constant-ints.ll
index 6b616aa700330a9..9568b155af13ae9 100644
--- a/llvm/test/Transforms/ConstraintElimination/large-constant-ints.ll
+++ b/llvm/test/Transforms/ConstraintElimination/large-constant-ints.ll
@@ -96,6 +96,7 @@ else:
   ret i1 false
 }
 
+; TODO: This could be folded.
 define i1 @sub_decomp_i80(i80 %a) {
 ; CHECK-LABEL: @sub_decomp_i80(
 ; CHECK-NEXT:  entry:
@@ -104,7 +105,8 @@ define i1 @sub_decomp_i80(i80 %a) {
 ; CHECK-NEXT:    br i1 [[C]], label [[THEN:%.*]], label [[ELSE:%.*]]
 ; CHECK:       then:
 ; CHECK-NEXT:    [[SUB_1:%.*]] = sub nuw i80 [[A]], 1973801615886922022913
-; CHECK-NEXT:    ret i1 true
+; CHECK-NEXT:    [[C_1:%.*]] = icmp ult i80 [[SUB_1]], 1346612317380797267967
+; CHECK-NEXT:    ret i1 [[C_1]]
 ; CHECK:       else:
 ; CHECK-NEXT:    ret i1 false
 ;
@@ -418,12 +420,12 @@ entry:
   ret i1 %res
 }
 
-; FIXME: This is a miscompile.
 define i1 @pr68751(i128 %arg) {
 ; CHECK-LABEL: @pr68751(
 ; CHECK-NEXT:    [[SHL1:%.*]] = shl nuw nsw i128 [[ARG:%.*]], 32
 ; CHECK-NEXT:    [[SHL2:%.*]] = shl nuw nsw i128 [[SHL1]], 32
-; CHECK-NEXT:    ret i1 true
+; CHECK-NEXT:    [[CMP:%.*]] = icmp eq i128 [[SHL2]], 0
+; CHECK-NEXT:    ret i1 [[CMP]]
 ;
   %shl1 = shl nuw nsw i128 %arg, 32
   %shl2 = shl nuw nsw i128 %shl1, 32
diff --git a/llvm/test/Transforms/ConstraintElimination/shl.ll b/llvm/test/Transforms/ConstraintElimination/shl.ll
index 982e0e745833351..9f98a9d3a57caef 100644
--- a/llvm/test/Transforms/ConstraintElimination/shl.ll
+++ b/llvm/test/Transforms/ConstraintElimination/shl.ll
@@ -1277,7 +1277,8 @@ define i1 @shl_55() {
 ; CHECK-LABEL: @shl_55(
 ; CHECK-NEXT:  entry:
 ; CHECK-NEXT:    [[SHL_UB:%.*]] = shl nuw nsw i256 1, 55
-; CHECK-NEXT:    ret i1 true
+; CHECK-NEXT:    [[SHL_CMP:%.*]] = icmp uge i256 [[SHL_UB]], 1
+; CHECK-NEXT:    ret i1 [[SHL_CMP]]
 ;
 entry:
   %shl.ub = shl nuw nsw i256 1, 55

[tru]

@nikic you put this in the 17.x milestone, should we try to get that into 17.0.3 tomorrow? Maybe optimistic since it's not even in main yet.

[fhahn]

LGTM, thanks!