Re: [PATCH] support to flip modes in sh mode-switching

[Uros Bizjak <ubizjak at gmail dot com>]

Hello Christian!

> This optimization is useful for the sh4-300. I don't know any other 
> architecture supported by gcc to test it. It also shares the same 
> module than the i386 targets to support mode switching.

I know one ;)

Looking briefly at your patch, it seems to me that it also implements 
the infrastructure needed for reliable x87<->MMX mode switching. As 
described in PR target/19161 (comment #16 and #17), the patch that used 
mode switching, failed for certain CFGs. For certain CFG layouts, it was 
possible to enter a BB with both (MMX and X87) modes active. The patch 
was designed to ICE for this situation, as both register sets were 
disabled in this case. However, having mode _flipping_ patch, this 
situation can be resolved.

> Bootstrapped the compiler and tested for no regression on 
> i686-pc-linux-gnu check-gcc. If there are specific flags to stress 387 
> modes I would be happy to run more tests.

You could create some loops and branches, where floor(), ceil() and 
perhaps trunc() functions are used. Try to compile it with -O2 
-ffast-math and this should exercise x87 mode switching stuff.

And BTW (looking at PR 28764, comment #12 by Jorn): x87 does not insert 
mode _switching_ code, but inserts code where mode control words are 
calculated. The reason that separate entities are used is due to 
"combining" of code that sets control bits. We can calculate a couple of 
control words at _one_ insertion point and use these words in actual 
mode switch:

--cut here--
double test(double *a, int x)
{
 int i;
 double z = 0.0;

 for (i = 0; i < x; i++)
   z += floor(a[i]) + ceil(a[i]);
 return z;
}
--cut here--

Compile this with -O2 -ffast-math to see that only ONE fstcw is used. 
Don't bother with fldcw insns...

> 	* basic-block.h (pre_edge_lcm_avs): Declare
> 	* config/i386/i386.h (EMIT_MODE_SET): Add ACTUALMODE parameter.
> 	* doc/tm.texi (EMIT_MODE_SET): Idem.
> 	* config/i386/sh.h (EMIT_MODE_SET): Idem. Call emit_fpu_flip.
IMO "* config/sh/sh.h" in the last line.

Uros.