On Wed, Dec 14, 2022 at 3:21 AM liuhongt via Gcc-patches
<gcc-patches@gcc.gnu.org> wrote:
>
> Don't add crtfastmath.o for -shared to avoid changing the MXCSR
> register when loading a shared library. crtfastmath.o will be used
> only when building executables.
>
> Bootstrapped and regtested on x86_64-pc-linux-gnu{-m32,}.
> Ok for trunk?
You reject negative -mdaz-ftz but wouldn't that be useful with
-Ofast -mno-daz-ftz since there's otherwise no way to avoid that?
Richard.
> gcc/ChangeLog:
>
> PR target/55522
> PR target/36821
> * config/i386/gnu-user-common.h (GNU_USER_TARGET_MATHFILE_SPEC):
> Link crtfastmath.o when -mdaz-ftz is specified, not link it
> when -shared is specified.
> * config/i386/i386.opt (mdaz-ftz): New option.
> * doc/invoke.texi (x86 options): Document mftz-daz.
> ---
> gcc/config/i386/gnu-user-common.h | 2 +-
> gcc/config/i386/i386.opt | 4 ++++
> gcc/doc/invoke.texi | 10 +++++++++-
> 3 files changed, 14 insertions(+), 2 deletions(-)
>
> diff --git a/gcc/config/i386/gnu-user-common.h
> b/gcc/config/i386/gnu-user-common.h
> index cab9be2bfb7..02e4a2192a4 100644
> --- a/gcc/config/i386/gnu-user-common.h
> +++ b/gcc/config/i386/gnu-user-common.h
> @@ -47,7 +47,7 @@ along with GCC; see the file COPYING3. If not see
>
> /* Similar to standard GNU userspace, but adding -ffast-math support. */
> #define GNU_USER_TARGET_MATHFILE_SPEC \
> - "%{Ofast|ffast-math|funsafe-math-optimizations:crtfastmath.o%s} \
> +
> "%{Ofast|ffast-math|funsafe-math-optimizations|mdaz-ftz:%{!shared:crtfastmath.o%s}}
> \
> %{mpc32:crtprec32.o%s} \
> %{mpc64:crtprec64.o%s} \
> %{mpc80:crtprec80.o%s}"
> diff --git a/gcc/config/i386/i386.opt b/gcc/config/i386/i386.opt
> index fb4e57ada7c..8fd222db857 100644
> --- a/gcc/config/i386/i386.opt
> +++ b/gcc/config/i386/i386.opt
> @@ -420,6 +420,10 @@ mpc80
> Target RejectNegative
> Set 80387 floating-point precision to 80-bit.
>
> +mdaz-ftz
> +Target RejectNegative
> +Set the FTZ and DAZ Flags.
> +
> mpreferred-stack-boundary=
> Target RejectNegative Joined UInteger Var(ix86_preferred_stack_boundary_arg)
> Attempt to keep stack aligned to this power of 2.
> diff --git a/gcc/doc/invoke.texi b/gcc/doc/invoke.texi
> index cb40b38b73a..670e3767fbd 100644
> --- a/gcc/doc/invoke.texi
> +++ b/gcc/doc/invoke.texi
> @@ -1433,7 +1433,7 @@ See RS/6000 and PowerPC Options.
> -m96bit-long-double -mlong-double-64 -mlong-double-80 -mlong-double-128
> @gol
> -mregparm=@var{num} -msseregparm @gol
> -mveclibabi=@var{type} -mvect8-ret-in-mem @gol
> --mpc32 -mpc64 -mpc80 -mstackrealign @gol
> +-mpc32 -mpc64 -mpc80 -mdaz-ftz -mstackrealign @gol
> -momit-leaf-frame-pointer -mno-red-zone -mno-tls-direct-seg-refs @gol
> -mcmodel=@var{code-model} -mabi=@var{name} -maddress-mode=@var{mode} @gol
> -m32 -m64 -mx32 -m16 -miamcu -mlarge-data-threshold=@var{num} @gol
> @@ -32752,6 +32752,14 @@ are enabled by default; routines in such libraries
> could suffer significant
> loss of accuracy, typically through so-called ``catastrophic cancellation'',
> when this option is used to set the precision to less than extended
> precision.
>
> +@item -mdaz-ftz
> +@opindex mdaz-ftz
> +
> +the flush-to-zero (FTZ) and denormals-are-zero (DAZ) flags in the MXCSR
> register
> +are used to control floating-point calculations.SSE and AVX instructions
> +including scalar and vector instructions could benefit from enabling the FTZ
> +and DAZ flags when @option{-mdaz-ftz} is specified.
Maybe say that the MXCSR register is set at program start to achieve
this when the
flag is specified at _link_ time and say this switch is ignored when
-shared is specified?
> +
> @item -mstackrealign
> @opindex mstackrealign
> Realign the stack at entry. On the x86, the @option{-mstackrealign}
> --
> 2.27.0
>
