Re: Question on lower-subreg.c

[Ian Lance Taylor <iant at google dot com>]

On Thu, Jan 24, 2013 at 11:57 PM, Bin.Cheng <amker.cheng@gmail.com> wrote:
>
> I read code in lower-subreg.c and found GCC only split some of
> multi-word mode instructions, like load from memory into pseudo reg,
> etc. The related code is in find_decomposable_subregs.
>
> So for below example from PR56102:
>
> double g = 1.0;
> double func(int a, double d)
> {
>     if (a > 0)
>     return 0.0 + g;
>     else
>     return 2.0 + d;
> }
>
> compiling with:
> ./arm-none-eabi-gcc -mthumb -mcpu=cortex-m0 -Os test.c -S -o test.S
>
> If split is done for all DFmode insns, it will save us code size by
> sharing constant pool in program.

On most platforms that would hurt performance, because most platforms
have a single instruction to load and store a DFmode value into a
floating point register.  In fact I suspect that transformation would
be a poor choice on any target that has 64-bit floating point
registers at all.

it would be a reasonable optimization for a target that has no
floating point registers and that stores all DFmode values as a pair
of 32-bit registers.  If you care about such a target, it would be
reasonable to add support to lower-subreg for that case.

Ian