[PATCH] PR target/65240, Fix Power{7,8} insn constraint issue with -O3 -ffast-math
Richard Biener
richard.guenther@gmail.com
Fri Mar 6 12:05:00 GMT 2015
On Thu, Mar 5, 2015 at 9:06 PM, Michael Meissner
<meissner@linux.vnet.ibm.com> wrote:
> This patch fixes PR 65240, which was a latent bug that was introduced when I
> added the -mupper-regs support to the PowerPC compiler. In the PowerPC
> compiler, if you use -ffast-math, the compiler allows floating point constants
> in move RTLs until register allocation time, in order to allow the
> optimizations that replace division by a constant with multiplication by the
> reciprocal.
Don't we perform this optimization on the GIMPLE/tree level already?
Richard.
> If the register being loaded up is a traditional Altivec register,
> the load will fail, since there is no D-FORM (register+offset) addressing mode
> for the traditional altivec registers.
>
> I had had a define_split to try and handle this situation, but it didn't work
> all of the time. I rewrote the insns, so there is an UNSPEC in it to prevent
> over optimization.
>
> I have done bootstrap builds and make check's on both a big endian power7
> system, and a little endian power8 system with no regressions in the tests. I
> have also built both power7 and power8 spec 2006 (v5) versions, and Spec
> built. I have run the Spec floating point tests on a power7 box, and all of
> the tests performed about at the same speed as before the bug fix was done,
> with the exception of gromacs, which is 4% faster with the fix.
>
> Looking at the static instruction counts, the big change is to fold the addi
> instruction into the load of the constant if it is loading the value to a
> traditional floating point register (previously, it would load up the constant
> with addis/addi and then do an indirect load). I do see other instructions
> change, including a few more floating point loads, but with the exception of
> gromacs, most of the changes are in the noise level.
>
> Is this ok to install into the trunk? At present, it is not needed for GCC
> 4.9, since that branch does not have the -mupper-regs support.
>
> [gcc]
> 2015-03-05 Michael Meissner <meissner@linux.vnet.ibm.com>
>
> PR target/65240
> * config/rs6000/rs6000.md (UNSPEC_FUSION_FP_CONSTANT): New unspec.
> (Vsx_load): New mode attribute to give appropriate VSX load
> instruction.
> (move floating point constant define_split): Use an UNSPEC to make
> sure that the load of the constant is kept as a memory address,
> instead of being converted back into a move of the constant.
> (load_<mode>_constant): Likewise.
>
> [gcc/testsuite]
> 2015-03-05 Michael Meissner <meissner@linux.vnet.ibm.com>
>
> PR target/65240
> * gcc.target/powerpc/pr65240.c: New test.
>
> --
> Michael Meissner, IBM
> IBM, M/S 2506R, 550 King Street, Littleton, MA 01460-6245, USA
> email: meissner@linux.vnet.ibm.com, phone: +1 (978) 899-4797
More information about the Gcc-patches
mailing list