[clang] Make fp contract(on) not overwrite earlier contract(fast) and CLI --ffp-contract=fast(-honor-pragmas) #169859

mikolaj-pirog · 2025-11-27T22:02:20Z

As in title. These are 2 changes to how fp contract pragmas interact with each other and the CLI options:

Code like this, compiled under -ffp-contract=fast

// 
float foo(float a, float b, float c, float d, float e)  {

    #pragma STDC FP_CONTRACT ON
    float mul = c * d;
    float add = mul + e;

   return add; 
}

will now have 'contract' set on fp ops, i.e. on pragma won't overwrite fast CLI option.

Code like this:

float bar(float a, float b, float c, float d, float e)  {
    #pragma clang fp contract(fast)
    #pragma clang fp contract(on)
    float mul = c * d;
    float add = mul + e;

   return add; 
}

will now have 'contract' set as well -- on doesn't overwrite previous fast.

…ragma-overwrite

llvmbot · 2025-11-27T22:02:53Z

@llvm/pr-subscribers-clang

Author: Mikołaj Piróg (mikolaj-pirog)

Changes

As in title. These are 2 changes to how fp contract pragmas interact with each other and the CLI options:

Code like this, compiled under -ffp-contract=fast

// 
float foo(float a, float b, float c, float d, float e)  {

    #pragma STDC FP_CONTRACT ON
    float mul = c * d;
    float add = mul + e;

   return add; 
}

```cpp

will now have 'contract' set on fp ops, i.e. `on` pragma won't overwrite `fast` CLI option.

- Code like this:

float bar(float a, float b, float c, float d, float e) {
#pragma clang fp contract(fast)
#pragma clang fp contract(on)
float mul = c * d;
float add = mul + e;

return add;
}

will now have 'contract' set as well -- `on` doesn't overwrite previous `fast`.






---
Full diff: https://github.com/llvm/llvm-project/pull/169859.diff


5 Files Affected:

- (modified) clang/lib/Sema/SemaAttr.cpp (+7-1) 
- (modified) clang/test/CodeGen/ffp-contract-fhp-pragma-override.cpp (+12-9) 
- (modified) clang/test/CodeGen/fp-contract-fast-pragma.cpp (+15-4) 
- (modified) clang/test/CodeGen/fp-function-attrs.cpp (+1-1) 
- (modified) clang/test/CodeGenCUDA/fp-contract.cu (+5-7) 


``````````diff
diff --git a/clang/lib/Sema/SemaAttr.cpp b/clang/lib/Sema/SemaAttr.cpp
index 7729c113e422e..8a9a1e62770e1 100644
--- a/clang/lib/Sema/SemaAttr.cpp
+++ b/clang/lib/Sema/SemaAttr.cpp
@@ -1381,9 +1381,15 @@ void Sema::ActOnPragmaVisibility(const IdentifierInfo* VisType,
 void Sema::ActOnPragmaFPContract(SourceLocation Loc,
                                  LangOptions::FPModeKind FPC) {
   FPOptionsOverride NewFPFeatures = CurFPFeatureOverrides();
+  bool HasContractOverride = NewFPFeatures.hasFPContractModeOverride();
+  auto DefContractMode = getLangOpts().getDefaultFPContractMode();
+
   switch (FPC) {
   case LangOptions::FPM_On:
-    NewFPFeatures.setAllowFPContractWithinStatement();
+    if ((HasContractOverride &&
+         NewFPFeatures.getFPContractModeOverride() == LangOptions::FPM_Off) ||
+        (!HasContractOverride && DefContractMode == LangOptions::FPM_Off))
+      NewFPFeatures.setAllowFPContractWithinStatement();
     break;
   case LangOptions::FPM_Fast:
   case LangOptions::FPM_FastHonorPragmas:
diff --git a/clang/test/CodeGen/ffp-contract-fhp-pragma-override.cpp b/clang/test/CodeGen/ffp-contract-fhp-pragma-override.cpp
index ff35c9204c79c..fe2d52fae5cf0 100644
--- a/clang/test/CodeGen/ffp-contract-fhp-pragma-override.cpp
+++ b/clang/test/CodeGen/ffp-contract-fhp-pragma-override.cpp
@@ -2,15 +2,16 @@
 
 float fp_contract_on_1(float a, float b, float c) {
   // CHECK-LABEL: fp_contract_on_1fff(
-  // CHECK: call float @llvm.fmuladd.f32(float {{.*}}, float {{.*}}, float {{.*}})
+  // CHECK: fmul contract float
+  // CHECK: fadd contract float
   #pragma STDC FP_CONTRACT ON
   return a * b + c;
 }
 
 float fp_contract_on_2(float a, float b, float c) {
   // CHECK-LABEL: fp_contract_on_2fff(
-  // CHECK: fmul float
-  // CHECK: fadd float
+  // CHECK: fmul contract float
+  // CHECK: fadd contract float
   #pragma STDC FP_CONTRACT ON
   float t = a * b;
   return t + c;
@@ -52,15 +53,16 @@ float fp_contract_default_2(float a, float b, float c) {
 
 float fp_contract_clang_on_1(float a, float b, float c) {
   // CHECK-LABEL: fp_contract_clang_on_1fff(
-  // CHECK: call float @llvm.fmuladd.f32(float {{.*}}, float {{.*}}, float {{.*}})
+  // CHECK: fmul contract float
+  // CHECK: fadd contract float
   #pragma clang fp contract(on)
   return a * b + c;
 }
 
 float fp_contract_clang_on_2(float a, float b, float c) {
   // CHECK-LABEL: fp_contract_clang_on_2fff(
-  // CHECK: fmul float
-  // CHECK: fadd float
+  // CHECK: fmul contract float
+  // CHECK: fadd contract float
   #pragma clang fp contract(on)
   float t = a * b;
   return t + c;
@@ -104,14 +106,15 @@ float fp_contract_clang_fast_2(float a, float b, float c) {
 
 float fp_contract_global_on_1(float a, float b, float c) {
   // CHECK-LABEL: fp_contract_global_on_1fff(
-  // CHECK: call float @llvm.fmuladd.f32(float {{.*}}, float {{.*}}, float {{.*}})
+  // CHECK: fmul contract float
+  // CHECK: fadd contract float
   return a * b + c;
 }
 
 float fp_contract_global_on_2(float a, float b, float c) {
   // CHECK-LABEL: fp_contract_global_on_2fff(
-  // CHECK: fmul float
-  // CHECK: fadd float
+  // CHECK: fmul contract float
+  // CHECK: fadd contract float
   float t = a * b;
   return t + c;
 }
diff --git a/clang/test/CodeGen/fp-contract-fast-pragma.cpp b/clang/test/CodeGen/fp-contract-fast-pragma.cpp
index 0bb01d6e17a1d..f0399dbdc9039 100644
--- a/clang/test/CodeGen/fp-contract-fast-pragma.cpp
+++ b/clang/test/CodeGen/fp-contract-fast-pragma.cpp
@@ -87,17 +87,28 @@ float fp_contract_6(float a, float b, float c) {
   return a * b + c;
 }
 
+// 'on' after 'fast' shouldn't disable 'fast'
+#pragma clang fp contract(fast)
+#pragma clang fp contract(on)
+float fp_contract_7(float a, float b, float c) {
+  // COMMON: _Z13fp_contract_7fff
+  // CHECK: %[[M:.+]] = fmul contract float %a, %b
+  // CHECK-NEXT: fadd contract float %[[M]], %c
+  // STRICT: %[[M:.+]] = tail call contract float @llvm.experimental.constrained.fmul.f32(float %a, float %b, metadata !"round.tonearest", metadata !"fpexcept.strict")
+  // STRICT-NEXT: tail call contract float @llvm.experimental.constrained.fadd.f32(float %[[M]], float %c, metadata !"round.tonearest", metadata !"fpexcept.strict")
+  return a * b + c;
+}
 
 #pragma clang fp contract(fast)
-float fp_contract_7(float a) {
-// COMMON: _Z13fp_contract_7f
+float fp_contract_8(float a) {
+// COMMON: _Z13fp_contract_8f
 // CHECK: tail call contract float @llvm.sqrt.f32(float %a)
 // STRICT: tail call contract float @llvm.experimental.constrained.sqrt.f32(float %a, metadata !"round.tonearest", metadata !"fpexcept.strict")
   return __builtin_sqrtf(a);
 }
 
-float fp_contract_8(float a) {
-// COMMON: _Z13fp_contract_8f
+float fp_contract_9(float a) {
+// COMMON: _Z13fp_contract_9f
 // CHECK: tail call float @llvm.sqrt.f32(float %a)
 // STRICT: tail call float @llvm.experimental.constrained.sqrt.f32(float %a, metadata !"round.tonearest", metadata !"fpexcept.strict")
 #pragma clang fp contract(off)
diff --git a/clang/test/CodeGen/fp-function-attrs.cpp b/clang/test/CodeGen/fp-function-attrs.cpp
index 3775bd5452d78..3b3122f492fc1 100644
--- a/clang/test/CodeGen/fp-function-attrs.cpp
+++ b/clang/test/CodeGen/fp-function-attrs.cpp
@@ -51,7 +51,7 @@ float test_contract_on_pragma(float a, float b, float c) {
 
 // CHECK: define{{.*}} float @_Z23test_contract_on_pragmafff(float noundef nofpclass(nan inf) %a, float noundef nofpclass(nan inf) %b, float noundef nofpclass(nan inf) %c)
 // CHECK: fmul fast float {{%.+}}, {{%.+}}
-// CHECK: fadd reassoc nnan ninf nsz arcp afn float {{%.+}}, {{%.+}}
+// CHECK: fadd fast float {{%.+}}, {{%.+}}
 
 // CHECK: attributes [[FAST_ATTRS]] = { {{.*}}"no-infs-fp-math"="true" {{.*}}"no-nans-fp-math"="true" "no-signed-zeros-fp-math"="true"{{.*}} }
 // CHECK: attributes [[PRECISE_ATTRS]] = { {{.*}}"no-infs-fp-math"="false" {{.*}}"no-nans-fp-math"="false" "no-signed-zeros-fp-math"="false"{{.*}} }
diff --git a/clang/test/CodeGenCUDA/fp-contract.cu b/clang/test/CodeGenCUDA/fp-contract.cu
index d6c796a817cbf..b3fc8e30e8990 100644
--- a/clang/test/CodeGenCUDA/fp-contract.cu
+++ b/clang/test/CodeGenCUDA/fp-contract.cu
@@ -268,8 +268,7 @@ __host__ __device__ float func2(float a, float b, float c) {
 // COMMON-LABEL: _Z5func3fff
 // NV-OPT-FAST: fma.rn.f32
 // NV-OPT-FAST-NEXT: st.param.b32
-// NV-OPT-FASTSTD: mul.rn.f32
-// NV-OPT-FASTSTD: add.rn.f32
+// NV-OPT-FASTSTD: fma.rn.f32
 // NV-OPT-FASTSTD-NEXT: st.param.b32
 // NV-OPT-ON: mul.rn.f32
 // NV-OPT-ON: add.rn.f32
@@ -278,8 +277,8 @@ __host__ __device__ float func2(float a, float b, float c) {
 // NV-OPT-OFF: add.rn.f32
 // NV-OPT-OFF-NEXT: st.param.b32
 
-// AMD-OPT-FAST-IR: fmul float
-// AMD-OPT-FAST-IR: fadd float
+// AMD-OPT-FAST-IR: fmul contract float
+// AMD-OPT-FAST-IR: fadd contract float
 // AMD-OPT-ON-IR: fmul float
 // AMD-OPT-ON-IR: fadd float
 // AMD-OPT-OFF-IR: fmul float
@@ -287,9 +286,8 @@ __host__ __device__ float func2(float a, float b, float c) {
 
 // AMD-OPT-FAST: v_fmac_f32_e32
 // AMD-OPT-FAST-NEXT: s_setpc_b64
-// AMD-OPT-FASTSTD: v_mul_f32_e32
-// AMD-OPT-FASTSTD-NEXT: v_add_f32_e32
-// AMD-OPT-FASTSTD-NEXT: s_setpc_b64
+// AMD-OPT-FASTSTD: v_fmac_f32_e32
+// AMD-OPT-FASTSTD: s_setpc_b64
 // AMD-OPT-ON: v_mul_f32_e32
 // AMD-OPT-ON-NEXT: v_add_f32_e32
 // AMD-OPT-ON-NEXT: s_setpc_b64

mikolaj-pirog · 2025-11-27T22:11:45Z

My understanding of interactions of this flag is based on treating on as a subset of fast -- so when we enable on, while under fast (via earlier pragma or CLI), we shouldn't overwrite effects of fast. Current behavior is surprising to me

mikolaj-pirog added 4 commits September 30, 2025 15:57

init

cdbf16c

Merge remote-tracking branch 'other/main' into mpirog/fix-fast-math-p…

f606693

…ragma-overwrite

Further work

0044a18

Format

c4cb1d1

mikolaj-pirog requested review from andykaylor, jcranmer-intel and paperchalice November 27, 2025 22:02

llvmbot added clang Clang issues not falling into any other category clang:frontend Language frontend issues, e.g. anything involving "Sema" labels Nov 27, 2025

mikolaj-pirog added the floating-point Floating-point math label Nov 27, 2025

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[clang] Make fp contract(on) not overwrite earlier contract(fast) and CLI --ffp-contract=fast(-honor-pragmas) #169859

[clang] Make fp contract(on) not overwrite earlier contract(fast) and CLI --ffp-contract=fast(-honor-pragmas) #169859

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