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Fix `unsafe-fp-math` attribute emission. 

The conditions for which Clang emits the `unsafe-fp-math` function
attribute has been modified as part of
`84a9ec2ff1ee97fd7e8ed988f5e7b197aab84a7`.
In the backend code generators `"unsafe-fp-math"="true"` enable floating
point contraction for the whole function.
The intent of the change in `84a9ec2ff1ee97fd7e8ed988f5e7b197aab84a7`
was to prevent backend code generators performing contractions when that
is not expected.
However the change is inaccurate and incomplete because it allows
`unsafe-fp-math` to be set also when only in-statement contraction is
allowed.

Consider the following example
```
float foo(float a, float b, float c) {
  float tmp = a * b;
  return tmp + c;
}
```
and compile it with the command line
```
clang -fno-math-errno -funsafe-math-optimizations -ffp-contract=on \
  -O2 -mavx512f -S -o -
```
The resulting assembly has a `vfmadd213ss` instruction which corresponds
to a fused multiply-add. From the user perspective there shouldn't be
any contraction because the multiplication and the addition are not in
the same statement.

The optimized IR is:
```
define float @test(float noundef %a, float noundef %b, float noundef %c) #0 {
  %mul = fmul reassoc nsz arcp afn float %b, %a
  %add = fadd reassoc nsz arcp afn float %mul, %c
  ret float %add
}

attributes #0 = {
  [...]
  "no-signed-zeros-fp-math"="true"
  "no-trapping-math"="true"
  [...]
  "unsafe-fp-math"="true"
}
```
The `"unsafe-fp-math"="true"` function attribute allows the backend code
generator to perform `(fadd (fmul a, b), c) -> (fmadd a, b, c)`.

In the current IR representation there is no way to determine the
statement boundaries from the original source code.
Because of this for in-statement only contraction the generated IR
doesn't have instructions with the `contract` fast-math flag and
`llvm.fmuladd` is being used to represent contractions opportunities
that occur within a single statement.
Therefore `"unsafe-fp-math"="true"` can only be emitted when contraction
across statements is allowed.

Moreover the change in `84a9ec2ff1ee97fd7e8ed988f5e7b197aab84a7` doesn't
take into account that the floating point math function attributes can
be refined during IR code generation of a function to handle the cases
where the floating point math options are modified within a compound
statement via pragmas (see `CGFPOptionsRAII`).
For consistency `unsafe-fp-math` needs to be disabled if the contraction
mode for any scope/operation is not `fast`.
Similarly for consistency reason the initialization of `UnsafeFPMath` of
in `TargetOptions` for the backend code generation should take into
account the contraction mode as well.

Reviewed By: zahiraam

Differential Revision: https://reviews.llvm.org/D136786
This commit is contained in:
Michele Scandale 2022-11-11 12:06:29 -08:00
parent 9044226ba2
commit b7d7c448df
5 changed files with 55 additions and 18 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

7
clang/lib/CodeGen/BackendUtil.cpp
View File

@ -409,7 +409,12 @@ static bool initTargetOptions(DiagnosticsEngine &Diags,
Options.NoInfsFPMath = LangOpts.NoHonorInfs;
Options.NoNaNsFPMath = LangOpts.NoHonorNaNs;
Options.NoZerosInBSS = CodeGenOpts.NoZeroInitializedInBSS;
Options.UnsafeFPMath = LangOpts.UnsafeFPMath;
Options.UnsafeFPMath = LangOpts.AllowFPReassoc && LangOpts.AllowRecip &&
LangOpts.NoSignedZero && LangOpts.ApproxFunc &&
(LangOpts.getDefaultFPContractMode() ==
LangOptions::FPModeKind::FPM_Fast ||
LangOpts.getDefaultFPContractMode() ==
LangOptions::FPModeKind::FPM_FastHonorPragmas);
Options.ApproxFuncFPMath = LangOpts.ApproxFunc;
Options.BBSections =

11
clang/lib/CodeGen/CGCall.cpp
View File

@ -1862,11 +1862,12 @@ void CodeGenModule::getDefaultFunctionAttributes(StringRef Name,
FuncAttrs.addAttribute("no-nans-fp-math", "true");
if (LangOpts.ApproxFunc)
FuncAttrs.addAttribute("approx-func-fp-math", "true");
if ((LangOpts.FastMath ||
(!LangOpts.FastMath && LangOpts.AllowFPReassoc &&
LangOpts.AllowRecip && !LangOpts.FiniteMathOnly &&
LangOpts.NoSignedZero && LangOpts.ApproxFunc)) &&
LangOpts.getDefaultFPContractMode() != LangOptions::FPModeKind::FPM_Off)
if (LangOpts.AllowFPReassoc && LangOpts.AllowRecip &&
LangOpts.NoSignedZero && LangOpts.ApproxFunc &&
(LangOpts.getDefaultFPContractMode() ==
LangOptions::FPModeKind::FPM_Fast ||
LangOpts.getDefaultFPContractMode() ==
LangOptions::FPModeKind::FPM_FastHonorPragmas))
FuncAttrs.addAttribute("unsafe-fp-math", "true");
if (CodeGenOpts.SoftFloat)
FuncAttrs.addAttribute("use-soft-float", "true");

9
clang/lib/CodeGen/CodeGenFunction.cpp
View File

@ -173,10 +173,11 @@ void CodeGenFunction::CGFPOptionsRAII::ConstructorHelper(FPOptions FPFeatures) {
mergeFnAttrValue("no-infs-fp-math", FPFeatures.getNoHonorInfs());
mergeFnAttrValue("no-nans-fp-math", FPFeatures.getNoHonorNaNs());
mergeFnAttrValue("no-signed-zeros-fp-math", FPFeatures.getNoSignedZero());
mergeFnAttrValue("unsafe-fp-math", FPFeatures.getAllowFPReassociate() &&
FPFeatures.getAllowReciprocal() &&
FPFeatures.getAllowApproxFunc() &&
FPFeatures.getNoSignedZero());
mergeFnAttrValue(
"unsafe-fp-math",
FPFeatures.getAllowFPReassociate() && FPFeatures.getAllowReciprocal() &&
FPFeatures.getAllowApproxFunc() && FPFeatures.getNoSignedZero() &&
FPFeatures.allowFPContractAcrossStatement());
}
CodeGenFunction::CGFPOptionsRAII::~CGFPOptionsRAII() {

18
clang/test/CodeGen/fp-function-attrs.cpp
View File

@ -1,4 +1,5 @@
// RUN: %clang_cc1 -triple x86_64-linux-gnu -ffast-math -ffp-contract=fast -emit-llvm -o - %s | FileCheck %s
// RUN: %clang_cc1 -triple x86_64-linux-gnu -ffast-math -ffp-contract=fast-honor-pragmas -emit-llvm -o - %s | FileCheck %s
float test_default(float a, float b, float c) {
float tmp = a;
@ -35,10 +36,23 @@ float test_reassociate_off_pragma(float a, float b, float c) {
return tmp;
}
// CHECK: define{{.*}} float @_Z27test_reassociate_off_pragmafff(float noundef %a, float noundef %b, float noundef %c) [[NOREASSOC_ATTRS:#[0-9]+]]
// CHECK: define{{.*}} float @_Z27test_reassociate_off_pragmafff(float noundef %a, float noundef %b, float noundef %c) [[NO_UNSAFE_ATTRS:#[0-9]+]]
// CHECK: fadd nnan ninf nsz arcp contract afn float {{%.+}}, {{%.+}}
// CHECK: fadd fast float {{%.+}}, {{%.+}}
float test_contract_on_pragma(float a, float b, float c) {
float tmp = a * b;
{
#pragma clang fp contract(on)
tmp += c;
}
return tmp;
}
// CHECK: define{{.*}} float @_Z23test_contract_on_pragmafff(float noundef %a, float noundef %b, float noundef %c) [[NO_UNSAFE_ATTRS:#[0-9]+]]
// CHECK: fmul fast float {{%.+}}, {{%.+}}
// CHECK: fadd reassoc nnan ninf nsz arcp afn float {{%.+}}, {{%.+}}
// CHECK: attributes [[FAST_ATTRS]] = { {{.*}}"no-infs-fp-math"="true" {{.*}}"no-nans-fp-math"="true" "no-signed-zeros-fp-math"="true" {{.*}}"unsafe-fp-math"="true"{{.*}} }
// CHECK: attributes [[PRECISE_ATTRS]] = { {{.*}}"no-infs-fp-math"="false" {{.*}}"no-nans-fp-math"="false" "no-signed-zeros-fp-math"="false" {{.*}}"unsafe-fp-math"="false"{{.*}} }
// CHECK: attributes [[NOREASSOC_ATTRS]] = { {{.*}}"no-infs-fp-math"="true" {{.*}}"no-nans-fp-math"="true" "no-signed-zeros-fp-math"="true" {{.*}}"unsafe-fp-math"="false"{{.*}} }
// CHECK: attributes [[NO_UNSAFE_ATTRS]] = { {{.*}}"no-infs-fp-math"="true" {{.*}}"no-nans-fp-math"="true" "no-signed-zeros-fp-math"="true" {{.*}}"unsafe-fp-math"="false"{{.*}} }

28
clang/test/CodeGen/func-attr.c
View File

@ -1,12 +1,28 @@
// RUN: %clang -c -target x86_64 -ffast-math \
// RUN: -emit-llvm -S -o - %s | FileCheck %s
// RUN: %clang_cc1 -triple x86_64-linux-gnu -ffast-math \
// RUN: -ffp-contract=fast -emit-llvm -o - %s | \
// RUN: FileCheck %s --check-prefixes=CHECK,CHECK-UNSAFE
// RUN: %clang -c -target x86_64 -funsafe-math-optimizations \
// RUN: -emit-llvm -S -o - %s | FileCheck %s
// RUN: %clang_cc1 -triple x86_64-linux-gnu -funsafe-math-optimizations \
// RUN: -ffp-contract=fast -emit-llvm -o - %s | \
// RUN: FileCheck %s --check-prefixes=CHECK,CHECK-UNSAFE
// RUN: %clang_cc1 -triple x86_64-linux-gnu -funsafe-math-optimizations \
// RUN: -ffp-contract=on -emit-llvm -o - %s | \
// RUN: FileCheck %s --check-prefixes=CHECK,CHECK-NOUNSAFE
// RUN: %clang_cc1 -triple x86_64-linux-gnu -funsafe-math-optimizations \
// RUN: -ffp-contract=off -emit-llvm -o - %s | \
// RUN: FileCheck %s --check-prefixes=CHECK,CHECK-NOUNSAFE
float foo(float a, float b) {
return a+b;
}
// CHECK: define{{.*}} float @foo(float noundef %{{.*}}, float noundef %{{.*}}){{.*}} [[FAST_ATTRS:#[0-9]+]]
// CHECK: attributes [[FAST_ATTRS]] = { {{.*}} "approx-func-fp-math"="true" {{.*}} "no-signed-zeros-fp-math"="true" "no-trapping-math"="true" {{.*}} "unsafe-fp-math"="true"
// CHECK: define{{.*}} float @foo(float noundef %{{.*}}, float noundef %{{.*}}){{.*}} [[ATTRS:#[0-9]+]]
// CHECK: attributes [[ATTRS]] = {
// CHECK-SAME: "approx-func-fp-math"="true"
// CHECK-SAME: "no-signed-zeros-fp-math"="true"
// CHECK-SAME: "no-trapping-math"="true"
// CHECK-UNSAFE-SAME: "unsafe-fp-math"="true"
// CHECK-NOUNSAFE-NOT: "unsafe-fp-math"="true"
// CHECK-SAME: }