[mlir][arith] Fix crash when constant-folding truncf of inf to FiniteOnly float type#186191
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…Only float type
When arith.truncf constant-folding converts a value such as infinity
to a type with fltNonfiniteBehavior::FiniteOnly (e.g. f4E2M1FN),
APFloat::convert hits an llvm_unreachable("semantics don't support inf\!").
The fix adds early-exit guards in convertFloatValue() to return failure()
before calling APFloat::convert() when the source value is infinity or NaN
and the target type cannot represent it. This makes the fold a no-op for
unrepresentable special values, matching the existing behavior for lossy
ordinary conversions.
Fixes llvm#185351
Assisted-by: Claude Code
Member
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@llvm/pr-subscribers-mlir Author: Mehdi Amini (joker-eph) ChangesWhen arith.truncf constant-folding converts a value such as infinity to a type with fltNonfiniteBehavior::FiniteOnly (e.g. f4E2M1FN), APFloat::convert hits an llvm_unreachable("semantics don't support inf!"). The fix adds early-exit guards in convertFloatValue() to return failure() before calling APFloat::convert() when the source value is infinity or NaN and the target type cannot represent it. This makes the fold a no-op for unrepresentable special values, matching the existing behavior for lossy ordinary conversions. Fixes #185351 Assisted-by: Claude Code Full diff: https://github.com/llvm/llvm-project/pull/186191.diff 2 Files Affected:
diff --git a/mlir/lib/Dialect/Arith/IR/ArithOps.cpp b/mlir/lib/Dialect/Arith/IR/ArithOps.cpp
index 6f368604df65a..d6a98a667da32 100644
--- a/mlir/lib/Dialect/Arith/IR/ArithOps.cpp
+++ b/mlir/lib/Dialect/Arith/IR/ArithOps.cpp
@@ -1451,6 +1451,17 @@ static bool checkWidthChangeCast(TypeRange inputs, TypeRange outputs) {
static FailureOr<APFloat> convertFloatValue(
APFloat sourceValue, const llvm::fltSemantics &targetSemantics,
llvm::RoundingMode roundingMode = llvm::RoundingMode::NearestTiesToEven) {
+ // Reject special values that are not representable in the target type before
+ // calling APFloat::convert, which would llvm_unreachable on them.
+ using fltNonfiniteBehavior = llvm::fltNonfiniteBehavior;
+ if (sourceValue.isInfinity() &&
+ (targetSemantics.nonFiniteBehavior == fltNonfiniteBehavior::NanOnly ||
+ targetSemantics.nonFiniteBehavior == fltNonfiniteBehavior::FiniteOnly))
+ return failure();
+ if (sourceValue.isNaN() &&
+ targetSemantics.nonFiniteBehavior == fltNonfiniteBehavior::FiniteOnly)
+ return failure();
+
bool losesInfo = false;
auto status = sourceValue.convert(targetSemantics, roundingMode, &losesInfo);
if (losesInfo || status != APFloat::opOK)
diff --git a/mlir/test/Dialect/Arith/canonicalize.mlir b/mlir/test/Dialect/Arith/canonicalize.mlir
index 26b04e4209a43..643e4e076e7c6 100644
--- a/mlir/test/Dialect/Arith/canonicalize.mlir
+++ b/mlir/test/Dialect/Arith/canonicalize.mlir
@@ -3527,3 +3527,29 @@ func.func @cmpi_dynamic_shape_no_fold(%arg0: tensor<?xi32>) -> tensor<?xi1> {
return %0 : tensor<?xi1>
}
+// -----
+
+// arith.truncf of infinity to a FiniteOnly float type (f4E2M1FN) must not fold,
+// since the type has no infinity representation. Previously this would crash
+// inside APFloat::convert with llvm_unreachable("semantics don't support inf!").
+
+// CHECK-LABEL: @truncf_inf_to_finite_only_no_fold
+// CHECK: arith.truncf
+func.func @truncf_inf_to_finite_only_no_fold() -> f4E2M1FN {
+ %inf = arith.constant 0x7F800000 : f32
+ %result = arith.truncf %inf : f32 to f4E2M1FN
+ return %result : f4E2M1FN
+}
+
+// -----
+
+// arith.truncf of negative infinity to a FiniteOnly float type must not fold.
+
+// CHECK-LABEL: @truncf_neg_inf_to_finite_only_no_fold
+// CHECK: arith.truncf
+func.func @truncf_neg_inf_to_finite_only_no_fold() -> f4E2M1FN {
+ %neg_inf = arith.constant 0xFF800000 : f32
+ %result = arith.truncf %neg_inf : f32 to f4E2M1FN
+ return %result : f4E2M1FN
+}
+
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Member
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@llvm/pr-subscribers-mlir-arith Author: Mehdi Amini (joker-eph) ChangesWhen arith.truncf constant-folding converts a value such as infinity to a type with fltNonfiniteBehavior::FiniteOnly (e.g. f4E2M1FN), APFloat::convert hits an llvm_unreachable("semantics don't support inf!"). The fix adds early-exit guards in convertFloatValue() to return failure() before calling APFloat::convert() when the source value is infinity or NaN and the target type cannot represent it. This makes the fold a no-op for unrepresentable special values, matching the existing behavior for lossy ordinary conversions. Fixes #185351 Assisted-by: Claude Code Full diff: https://github.com/llvm/llvm-project/pull/186191.diff 2 Files Affected:
diff --git a/mlir/lib/Dialect/Arith/IR/ArithOps.cpp b/mlir/lib/Dialect/Arith/IR/ArithOps.cpp
index 6f368604df65a..d6a98a667da32 100644
--- a/mlir/lib/Dialect/Arith/IR/ArithOps.cpp
+++ b/mlir/lib/Dialect/Arith/IR/ArithOps.cpp
@@ -1451,6 +1451,17 @@ static bool checkWidthChangeCast(TypeRange inputs, TypeRange outputs) {
static FailureOr<APFloat> convertFloatValue(
APFloat sourceValue, const llvm::fltSemantics &targetSemantics,
llvm::RoundingMode roundingMode = llvm::RoundingMode::NearestTiesToEven) {
+ // Reject special values that are not representable in the target type before
+ // calling APFloat::convert, which would llvm_unreachable on them.
+ using fltNonfiniteBehavior = llvm::fltNonfiniteBehavior;
+ if (sourceValue.isInfinity() &&
+ (targetSemantics.nonFiniteBehavior == fltNonfiniteBehavior::NanOnly ||
+ targetSemantics.nonFiniteBehavior == fltNonfiniteBehavior::FiniteOnly))
+ return failure();
+ if (sourceValue.isNaN() &&
+ targetSemantics.nonFiniteBehavior == fltNonfiniteBehavior::FiniteOnly)
+ return failure();
+
bool losesInfo = false;
auto status = sourceValue.convert(targetSemantics, roundingMode, &losesInfo);
if (losesInfo || status != APFloat::opOK)
diff --git a/mlir/test/Dialect/Arith/canonicalize.mlir b/mlir/test/Dialect/Arith/canonicalize.mlir
index 26b04e4209a43..643e4e076e7c6 100644
--- a/mlir/test/Dialect/Arith/canonicalize.mlir
+++ b/mlir/test/Dialect/Arith/canonicalize.mlir
@@ -3527,3 +3527,29 @@ func.func @cmpi_dynamic_shape_no_fold(%arg0: tensor<?xi32>) -> tensor<?xi1> {
return %0 : tensor<?xi1>
}
+// -----
+
+// arith.truncf of infinity to a FiniteOnly float type (f4E2M1FN) must not fold,
+// since the type has no infinity representation. Previously this would crash
+// inside APFloat::convert with llvm_unreachable("semantics don't support inf!").
+
+// CHECK-LABEL: @truncf_inf_to_finite_only_no_fold
+// CHECK: arith.truncf
+func.func @truncf_inf_to_finite_only_no_fold() -> f4E2M1FN {
+ %inf = arith.constant 0x7F800000 : f32
+ %result = arith.truncf %inf : f32 to f4E2M1FN
+ return %result : f4E2M1FN
+}
+
+// -----
+
+// arith.truncf of negative infinity to a FiniteOnly float type must not fold.
+
+// CHECK-LABEL: @truncf_neg_inf_to_finite_only_no_fold
+// CHECK: arith.truncf
+func.func @truncf_neg_inf_to_finite_only_no_fold() -> f4E2M1FN {
+ %neg_inf = arith.constant 0xFF800000 : f32
+ %result = arith.truncf %neg_inf : f32 to f4E2M1FN
+ return %result : f4E2M1FN
+}
+
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kuhar
approved these changes
Mar 12, 2026
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When arith.truncf constant-folding converts a value such as infinity to a type with fltNonfiniteBehavior::FiniteOnly (e.g. f4E2M1FN), APFloat::convert hits an llvm_unreachable("semantics don't support inf!").
The fix adds early-exit guards in convertFloatValue() to return failure() before calling APFloat::convert() when the source value is infinity or NaN and the target type cannot represent it. This makes the fold a no-op for unrepresentable special values, matching the existing behavior for lossy ordinary conversions.
Fixes #185351
Assisted-by: Claude Code