-ffp-contract=fast-honor-pragmas is a valid option recognized
by clang -cc1 but it is missed by clang driver.
Reviewed by: Matt Arsenault
Differential Revision: https://reviews.llvm.org/D145648
flag. But the driver checks for 'fno-math-errno' before passing
'funsafe-math-optimizations' to the FE. In GCC, the option
'funsafe-math-optimizations' doesn't affect the 'fmath-errno' flag.
This patch aligns clang with GCC.
'-ffast-math' sets the FPContract to 'fast'. But 'funsafe-math-optimizations'
the driver doesn't consider the FPContract when handling the option.
Unfortunately there are places in the BE that interpret unsafe math
mode as allowing FMA. This patch makes -ffast-math' and
'funsafe-math-optimizations' behave similarly in regard to the setting of the
FPContract.
Differential Revision: https://reviews.llvm.org/D137578
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
