[PATCH] rs6000: Allow FPRs to change between SDmode and DDmode [PR94254]

[Segher Boessenkool]

Hi!

On Mon, Mar 23, 2020 at 06:37:52PM +0000, Richard Sandiford wrote:
> g:497498c878d48754318e486428e2aa30854020b9 caused lra to cycle
> on some SDmode reloads for power6.  As explained in more detail
> in the PR comments, the problem was a conflict between two target
> hooks: rs6000_secondary_memory_needed_mode required SDmode FPR
> reloads to use DDmode memory (rightly, since using SDmode memory
> wouldn't make progress) but rs6000_can_change_mode_class didn't
> allow FPRs to change from SDmode to DDmode.  Previously lra
> ignored that and changed the mode anyway.
> 
> >From what Segher says, it sounds like the "from_size < 8 || to_size < 8"
> check is mostly there for SF<->64-bit subregs, and that SDmode is stored
> in the way that target-independent code expects.

That is what it looks like to me, yes.

> This patch therefore
> allows SD<->DD changes.
> 
> I wondered about checking for SD<->64-bit changes instead, but that
> seemed like an unnecessary generalisation for this stage.

Yeah, good call.

> Bootstrapped and regression-tested on powerpc64le-unknown-linux-gnu,
> both with default settings and with --with-cpu=power6.  (For power6
> I reverted the lra patch to do the before testing, since the build
> hanged otherwise.)  The patch fixed the regressing testcases and
> introduced no other test changes.  Zdenek confirms that it also
> fixes powerpc-linux-gnu builds.  OK to install?

(Already committed).

Thanks for doing the patch!


Segher


> gcc/
> 	PR target/94254
> 	* config/rs6000/rs6000.c (rs6000_can_change_mode_class): Allow
> 	FPRs to change between SDmode and DDmode.