[clang] 1b69ce1 - Currently the options ‘ffast-math’ and ‘ffp-contract’ are connected.
Zahira Ammarguellat via cfe-commits
cfe-commits at lists.llvm.org
Fri Sep 16 08:08:53 PDT 2022
Author: Zahira Ammarguellat
Date: 2022-09-16T11:08:41-04:00
New Revision: 1b69ce1208976c71bf7ee3932aa272462c1feb1b
URL: https://github.com/llvm/llvm-project/commit/1b69ce1208976c71bf7ee3932aa272462c1feb1b
DIFF: https://github.com/llvm/llvm-project/commit/1b69ce1208976c71bf7ee3932aa272462c1feb1b.diff
LOG: Currently the options ‘ffast-math’ and ‘ffp-contract’ are connected.
When ‘ffast-math’ is set, ffp-contract is altered this way:
-ffast-math/ Ofast -> ffp-contract=fast
-fno-fast-math -> if ffp-contract= fast then ffp-contract=on else
ffp-contract unchanged
This differs from gcc which doesn’t connect the two options.
Connecting these two options in clang, resulted in spurious warnings
when the user combines these two options -ffast-math -fno-fast-math; see
issue https://github.com/llvm/llvm-project/issues/54625.
The issue is that the ‘ffast-math’ option is an on/off flag, but the
‘ffp-contract’ is an on/off/fast flag. So when ‘fno-fast-math’ is used
there is no obvious value for ‘ffp-contract’. What should the value of
ffp-contract be for -ffp-contract=fast -fno-fast-math and -ffast-math
-ffp-contract=fast -fno-fast-math? The current logic sets ffp-contract
back to on in these cases. This doesn’t take into account that the value
of ffp-contract is modified by an explicit ffp-contract` option.
This patch is proposing a set of rules to apply when ffp-contract',
ffast-math and fno-fast-math are combined. These rules would give the
user the expected behavior and no diagnostic would be needed.
See RFC
https://discourse.llvm.org/t/rfc-making-ffast-math-option-unrelated-to-ffp-contract-option/61912
Added:
clang/test/Driver/fp-contract.c
Modified:
clang/docs/UsersManual.rst
clang/lib/Driver/ToolChains/Clang.cpp
clang/test/CodeGen/ffp-contract-option.c
Removed:
################################################################################
diff --git a/clang/docs/UsersManual.rst b/clang/docs/UsersManual.rst
index e9c1be4cdd0f5..bc107be41fab8 100644
--- a/clang/docs/UsersManual.rst
+++ b/clang/docs/UsersManual.rst
@@ -1359,8 +1359,8 @@ floating point semantic models: precise (the default), strict, and fast.
"fenv_access", "off", "on", "off"
"rounding_mode", "tonearest", "dynamic", "tonearest"
"contract", "on", "off", "fast"
- "denormal_fp_math", "IEEE", "IEEE", "PreserveSign"
- "denormal_fp32_math", "IEEE","IEEE", "PreserveSign"
+ "denormal_fp_math", "IEEE", "IEEE", "IEEE"
+ "denormal_fp32_math", "IEEE","IEEE", "IEEE"
"support_math_errno", "on", "on", "off"
"no_honor_nans", "off", "off", "on"
"no_honor_infinities", "off", "off", "on"
@@ -1408,6 +1408,61 @@ floating point semantic models: precise (the default), strict, and fast.
* ``-ffp-contract=fast``
+ Note: ``-ffast-math`` causes ``crtfastmath.o`` to be linked with code. See
+ :ref:`crtfastmath.o` for more details.
+
+.. option:: -fno-fast-math
+
+ Disable fast-math mode. This options disables unsafe floating-point
+ optimizations by preventing the compiler from making any tranformations that
+ could affect the results.
+
+ This option implies:
+
+ * ``-fhonor-infinities``
+
+ * ``-fhonor-nans``
+
+ * ``-fmath-errno``
+
+ * ``-fno-finite-math-only``
+
+ * ``-fno-associative-math``
+
+ * ``-fno-reciprocal-math``
+
+ * ``-fsigned-zeros``
+
+ * ``-fno-trapping-math``
+
+ * ``-ffp-contract=on``
+
+ * ``-fdenormal-fp-math=ieee``
+
+ There is ambiguity about how ``-ffp-contract``, ``-ffast-math``,
+ and ``-fno-fast-math`` behave in combination. To keep the value of
+ ``-ffp-contract`` consistent, we define this set of rules:
+
+ * ``-ffast-math`` sets ``ffp-contract`` to ``fast``.
+
+ * ``-fno-fast-math`` sets ``-ffp-contract`` to ``on`` (``fast`` for CUDA and
+ HIP).
+
+ * If ``-ffast-math`` and ``-ffp-contract`` are both seen, but
+ ``-ffast-math`` is not followed by ``-fno-fast-math``, ``ffp-contract``
+ will be given the value of whichever option was last seen.
+
+ * If ``-fno-fast-math`` is seen and ``-ffp-contract`` has been seen at least
+ once, the ``ffp-contract`` will get the value of the last seen value of
+ ``-ffp-contract``.
+
+ * If ``-fno-fast-math`` is seen and ``-ffp-contract`` has not been seen, the
+ ``-ffp-contract`` setting is determined by the default value of
+ ``-ffp-contract``.
+
+ Note: ``-fno-fast-math`` implies ``-fdenormal-fp-math=ieee``.
+ ``-fno-fast-math`` causes ``crtfastmath.o`` to not be linked with code.
+
.. option:: -fdenormal-fp-math=<value>
Select which denormal numbers the code is permitted to require.
@@ -1632,6 +1687,15 @@ Note that floating-point operations performed as part of constant initialization
has no effect because the optimizer is prohibited from making unsafe
transformations.
+.. _crtfastmath.o:
+
+A note about ``crtfastmath.o``
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+``-ffast-math`` and ``-funsafe-math-optimizations`` cause ``crtfastmath.o`` to be
+automatically linked, which adds a static constructor that sets the FTZ/DAZ
+bits in MXCSR, affecting not only the current compilation unit but all static
+and shared libraries included in the program.
+
.. _FLT_EVAL_METHOD:
A note about ``__FLT_EVAL_METHOD__``
diff --git a/clang/lib/Driver/ToolChains/Clang.cpp b/clang/lib/Driver/ToolChains/Clang.cpp
index e215dc06379b4..9cc08306471c2 100644
--- a/clang/lib/Driver/ToolChains/Clang.cpp
+++ b/clang/lib/Driver/ToolChains/Clang.cpp
@@ -2786,6 +2786,8 @@ static void RenderFloatingPointOptions(const ToolChain &TC, const Driver &D,
// CUDA and HIP don't rely on the frontend to pass an ffp-contract option.
// If one wasn't given by the user, don't pass it here.
StringRef FPContract;
+ StringRef LastSeenFfpContractOption;
+ bool SeenFfastMathOption = false;
if (!JA.isDeviceOffloading(Action::OFK_Cuda) &&
!JA.isOffloading(Action::OFK_HIP))
FPContract = "on";
@@ -2936,9 +2938,10 @@ static void RenderFloatingPointOptions(const ToolChain &TC, const Driver &D,
// -ffp-model=precise sets PreciseFPModel to on and Val to
// "precise". FPContract is set.
;
- } else if (Val.equals("fast") || Val.equals("on") || Val.equals("off"))
+ } else if (Val.equals("fast") || Val.equals("on") || Val.equals("off")) {
FPContract = Val;
- else
+ LastSeenFfpContractOption = Val;
+ } else
D.Diag(diag::err_drv_unsupported_option_argument)
<< A->getOption().getName() << Val;
break;
@@ -3031,6 +3034,7 @@ static void RenderFloatingPointOptions(const ToolChain &TC, const Driver &D,
RoundingFPMath = false;
// If fast-math is set then set the fp-contract mode to fast.
FPContract = "fast";
+ SeenFfastMathOption = true;
break;
case options::OPT_fno_fast_math:
HonorINFs = true;
@@ -3047,13 +3051,12 @@ static void RenderFloatingPointOptions(const ToolChain &TC, const Driver &D,
DenormalFPMath = DefaultDenormalFPMath;
DenormalFP32Math = llvm::DenormalMode::getIEEE();
if (!JA.isDeviceOffloading(Action::OFK_Cuda) &&
- !JA.isOffloading(Action::OFK_HIP))
- if (FPContract == "fast") {
+ !JA.isOffloading(Action::OFK_HIP)) {
+ if (LastSeenFfpContractOption != "") {
+ FPContract = LastSeenFfpContractOption;
+ } else if (SeenFfastMathOption)
FPContract = "on";
- D.Diag(clang::diag::warn_drv_overriding_flag_option)
- << "-ffp-contract=fast"
- << "-ffp-contract=on";
- }
+ }
break;
}
if (StrictFPModel) {
diff --git a/clang/test/CodeGen/ffp-contract-option.c b/clang/test/CodeGen/ffp-contract-option.c
index 209f593da2cbd..232403bf98369 100644
--- a/clang/test/CodeGen/ffp-contract-option.c
+++ b/clang/test/CodeGen/ffp-contract-option.c
@@ -35,7 +35,7 @@
// RUN: | FileCheck %s --check-prefixes=CHECK,CHECK-FPC-ON
// RUN: %clang -Xclang -no-opaque-pointers -S -emit-llvm -ffp-contract=fast -fno-fast-math \
-// RUN: %s -o - | FileCheck %s --check-prefixes=CHECK,CHECK-FPC-ON
+// RUN: %s -o - | FileCheck %s --check-prefixes=CHECK,CHECK-CONTRACTFAST
// RUN: %clang -Xclang -no-opaque-pointers -S -emit-llvm -ffp-contract=on -fno-fast-math \
// RUN: %s -o - | FileCheck %s --check-prefixes=CHECK,CHECK-FPC-ON
diff --git a/clang/test/Driver/fp-contract.c b/clang/test/Driver/fp-contract.c
new file mode 100644
index 0000000000000..c4d184003c42d
--- /dev/null
+++ b/clang/test/Driver/fp-contract.c
@@ -0,0 +1,114 @@
+// Test that -ffp-contract is set to the right value when combined with
+// the options -ffast-math and -fno-fast-math.
+
+// RUN: %clang -### -ffast-math -c %s 2>&1 \
+// RUN: | FileCheck --check-prefix=CHECK-FPC-FAST %s
+// CHECK-FPC-FAST: "-ffp-contract=fast"
+
+// RUN: %clang -### -fno-fast-math -c %s 2>&1 \
+// RUN: | FileCheck --check-prefix=CHECK-FPC-ON %s
+
+// RUN: %clang -### -ffast-math -ffp-contract=on -c %s 2>&1 \
+// RUN: | FileCheck --check-prefix=CHECK-FPC-ON %s
+// CHECK-FPC-ON: "-ffp-contract=on"
+
+// RUN: %clang -### -ffast-math -ffp-contract=off -c %s 2>&1 \
+// RUN: | FileCheck --check-prefix=CHECK-FPC-OFF %s
+// CHECK-FPC-OFF: "-ffp-contract=off"
+
+// RUN: %clang -### -ffast-math -ffp-contract=fast -c %s 2>&1 \
+// RUN: | FileCheck --check-prefix=CHECK-FPC-FAST %s
+
+// RUN: %clang -### -ffp-contract=fast -ffast-math -c %s 2>&1 \
+// RUN: | FileCheck --check-prefix=CHECK-FPC-FAST %s
+// RUN: %clang -### -ffp-contract=on -ffast-math -c %s 2>&1 \
+// RUN: | FileCheck --check-prefix=CHECK-FPC-FAST %s
+// RUN: %clang -### -ffp-contract=off -ffast-math -c %s 2>&1 \
+// RUN: | FileCheck --check-prefix=CHECK-FPC-FAST %s
+
+// RUN: %clang -### -ffp-contract=fast -fno-fast-math -c %s 2>&1 \
+// RUN: | FileCheck --check-prefix=CHECK-FPC-FAST %s
+// RUN: %clang -### -ffp-contract=on -fno-fast-math -c %s 2>&1 \
+// RUN: | FileCheck --check-prefix=CHECK-FPC-ON %s
+// RUN: %clang -### -ffp-contract=off -fno-fast-math -c %s 2>&1 \
+// RUN: | FileCheck --check-prefix=CHECK-FPC-OFF %s
+
+
+// RUN: %clang -### -ffast-math -ffp-contract=fast -ffp-contract=on -c %s 2>&1 \
+// RUN: | FileCheck --check-prefix=CHECK-FPC-ON %s
+
+// RUN: %clang -### -ffast-math -ffp-contract=on -ffp-contract=off -c %s 2>&1 \
+// RUN: | FileCheck --check-prefix=CHECK-FPC-OFF %s
+
+// RUN: %clang -### -ffast-math -ffp-contract=on -ffp-contract=fast -c %s 2>&1 \
+// RUN: | FileCheck --check-prefix=CHECK-FPC-FAST %s
+
+// RUN: %clang -### -ffast-math -ffp-contract=off -ffp-contract=on -c %s 2>&1 \
+// RUN: | FileCheck --check-prefix=CHECK-FPC-ON %s
+
+// RUN: %clang -### -ffast-math -ffp-contract=off -ffp-contract=fast \
+// RUN: -c %s 2>&1 | FileCheck --check-prefix=CHECK-FPC-FAST %s
+
+// RUN: %clang -### -ffast-math -ffp-contract=on -fno-fast-math -c %s 2>&1 \
+// RUN: | FileCheck --check-prefix=CHECK-FPC-ON %s
+
+// RUN: %clang -### -ffast-math -ffp-contract=off -fno-fast-math -c %s 2>&1 \
+// RUN: | FileCheck --check-prefix=CHECK-FPC-OFF %s
+
+// RUN: %clang -### -ffast-math -ffp-contract=fast -fno-fast-math -c %s 2>&1 \
+// RUN: | FileCheck --check-prefix=CHECK-FPC-FAST %s
+
+// RUN: %clang -### -ffast-math -fno-fast-math -c %s 2>&1 \
+// RUN: | FileCheck --check-prefix=CHECK-FPC-ON %s
+
+// RUN: %clang -### -fno-fast-math -ffast-math -c %s 2>&1 \
+// RUN: | FileCheck --check-prefix=CHECK-FPC-FAST %s
+
+// RUN: %clang -### -fno-fast-math -ffp-contract=on -c %s 2>&1 \
+// RUN: | FileCheck --check-prefix=CHECK-FPC-ON %s
+
+// RUN: %clang -### -fno-fast-math -ffp-contract=off -c %s 2>&1 \
+// RUN: | FileCheck --check-prefix=CHECK-FPC-OFF %s
+
+// RUN: %clang -### -fno-fast-math -ffp-contract=fast -c %s 2>&1 \
+// RUN: | FileCheck --check-prefix=CHECK-FPC-FAST %s
+
+// RUN: %clang -### -ffp-contract=fast -fno-fast-math -ffp-contract=on \
+// RUN: -c %s 2>&1 | FileCheck --check-prefix=CHECK-FPC-ON %s
+
+// RUN: %clang -### -ffp-contract=fast -fno-fast-math -ffp-contract=off \
+// RUN: -c %s 2>&1 | FileCheck --check-prefix=CHECK-FPC-OFF %s
+
+// RUN: %clang -### -ffp-contract=off -fno-fast-math -ffp-contract=fast \
+// RUN: -c %s 2>&1 | FileCheck --check-prefix=CHECK-FPC-FAST %s
+
+// RUN: %clang -### -ffp-contract=off -fno-fast-math -ffp-contract=on \
+// RUN: -c %s 2>&1 | FileCheck --check-prefix=CHECK-FPC-ON %s
+
+// RUN: %clang -### -ffp-contract=on -ffast-math -c %s 2>&1 \
+// RUN: | FileCheck --check-prefix=CHECK-FPC-FAST %s
+
+// RUN: %clang -### -ffp-contract=off -ffast-math -c %s 2>&1 \
+// RUN: | FileCheck --check-prefix=CHECK-FPC-FAST %s
+
+// RUN: %clang -### -ffp-contract=fast -ffast-math -c %s 2>&1 \
+// RUN: | FileCheck --check-prefix=CHECK-FPC-FAST %s
+
+// RUN: %clang -### -ffp-contract=on -ffast-math -fno-fast-math -c %s 2>&1 \
+// RUN: | FileCheck --check-prefix=CHECK-FPC-ON %s
+
+// RUN: %clang -### -ffp-contract=off -ffast-math -fno-fast-math -c %s 2>&1 \
+// RUN: | FileCheck --check-prefix=CHECK-FPC-OFF %s
+
+// RUN: %clang -### -ffp-contract=fast -ffast-math -fno-fast-math -c %s 2>&1 \
+// RUN: | FileCheck --check-prefix=CHECK-FPC-FAST %s
+
+// RUN: %clang -### -fno-fast-math -ffast-math -ffp-contract=fast \
+// RUN: -c %s 2>&1 | FileCheck --check-prefix=CHECK-FPC-FAST %s
+
+// RUN: %clang -### -fno-fast-math -ffast-math -ffp-contract=on \
+// RUN: -c %s 2>&1 | FileCheck --check-prefix=CHECK-FPC-ON %s
+
+// RUN: %clang -### -fno-fast-math -ffast-math -ffp-contract=off \
+// RUN: -c %s 2>&1 | FileCheck --check-prefix=CHECK-FPC-OFF %s
+
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