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[Flang][Driver] Deprecate Ofast #101701

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Merged
merged 2 commits into from
Jan 24, 2025
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[Flang][Driver] Deprecate Ofast #101701

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b33ae61
[Flang][Driver] Deprecate Ofast
kiranchandramohan Jul 19, 2024
88f7792
Changes for rebase
kiranchandramohan Jan 15, 2025
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4 changes: 4 additions & 0 deletions clang/include/clang/Basic/DiagnosticDriverKinds.td
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Original file line number Diff line number Diff line change
Expand Up @@ -452,6 +452,10 @@ def warn_drv_deprecated_arg_ofast : Warning<
"argument '-Ofast' is deprecated; use '-O3 -ffast-math' for the same behavior,"
" or '-O3' to enable only conforming optimizations">,
InGroup<DeprecatedOFast>;
def warn_drv_deprecated_arg_ofast_for_flang : Warning<
"argument '-Ofast' is deprecated; use '-O3 -ffast-math -fstack-arrays' for the same behavior,"
" or '-O3 -fstack-arrays' to enable only conforming optimizations">,
InGroup<DeprecatedOFast>;
def warn_drv_deprecated_custom : Warning<
"argument '%0' is deprecated, %1">, InGroup<Deprecated>;
def warn_drv_assuming_mfloat_abi_is : Warning<
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10 changes: 6 additions & 4 deletions clang/include/clang/Driver/Options.td
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Expand Up @@ -933,10 +933,12 @@ def O : Joined<["-"], "O">, Group<O_Group>,
def O_flag : Flag<["-"], "O">, Visibility<[ClangOption, CC1Option, FC1Option]>,
Alias<O>, AliasArgs<["1"]>;
def Ofast : Joined<["-"], "Ofast">, Group<O_Group>,
Visibility<[ClangOption, CC1Option, FlangOption]>,
HelpTextForVariants<[ClangOption, CC1Option],
"Deprecated; use '-O3 -ffast-math' for the same behavior,"
" or '-O3' to enable only conforming optimizations">;
Visibility<[ClangOption, CC1Option, FlangOption, FC1Option]>,
HelpTextForVariants<[FlangOption, FC1Option],
"Deprecated; use '-O3 -ffast-math -fstack-arrays' for the same behavior,"
" or '-O3 -fstack-arrays' to enable only conforming optimizations">,
Comment on lines +937 to +939
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@luporl luporl Jan 15, 2025

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This may be out of the scope of this change, but why isn't -fstack-arrays enabled with -O3?

If -fstack-arrays is not specified are array temporaries always allocated on the heap?

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@kiranchandramohan kiranchandramohan Jan 15, 2025

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This is currently mimicking gfortran (https://gcc.gnu.org/onlinedocs/gfortran/Code-Gen-Options.html), which runs it at Ofast.

We have not seen much benefits because Flang mostly allocates on the stack. @tblah is that right?

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@tblah tblah Jan 15, 2025

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I think we saw significant benefits on SNAP but not on SPEC2017 rate. I don't think we experimented with anything else.

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@luporl luporl Jan 15, 2025

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Makes sense, thanks for the explanations.

HelpText<"Deprecated; use '-O3 -ffast-math' for the same behavior,"
" or '-O3' to enable only conforming optimizations">;
def P : Flag<["-"], "P">,
Visibility<[ClangOption, CC1Option, FlangOption, FC1Option]>,
Group<Preprocessor_Group>,
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1 change: 1 addition & 0 deletions clang/lib/Driver/ToolChains/Flang.cpp
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Expand Up @@ -934,6 +934,7 @@ void Flang::ConstructJob(Compilation &C, const JobAction &JA,
D.Diag(diag::warn_O4_is_O3);
} else if (A->getOption().matches(options::OPT_Ofast)) {
CmdArgs.push_back("-O3");
D.Diag(diag::warn_drv_deprecated_arg_ofast_for_flang);
} else {
A->render(Args, CmdArgs);
}
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5 changes: 5 additions & 0 deletions flang/test/Driver/fast-math.f90
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@@ -1,6 +1,11 @@
! Test for correct forwarding of fast-math flags from the compiler driver to the
! frontend driver

! Check warning message for Ofast deprecation
! RUN: %flang -Ofast -### %s -o %t 2>&1 | FileCheck %s
! CHECK: warning: argument '-Ofast' is deprecated; use '-O3 -ffast-math -fstack-arrays' for the same behavior, or '-O3
! -fstack-arrays' to enable only conforming optimizations [-Wdeprecated-ofast]

! -Ofast => -ffast-math -O3 -fstack-arrays
! RUN: %flang -Ofast -fsyntax-only -### %s -o %t 2>&1 \
! RUN: | FileCheck --check-prefix=CHECK-OFAST %s
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