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Re: scheduler first pass and destructive scheduling of REG_LIBCALL/REG_RETVALblocks?
- To: Franz Sirl <Franz dot Sirl-kernel at lauterbach dot com>
- Subject: Re: scheduler first pass and destructive scheduling of REG_LIBCALL/REG_RETVALblocks?
- From: Bernd Schmidt <bernds at redhat dot com>
- Date: Mon, 2 Apr 2001 11:57:40 +0100 (BST)
- Cc: <gcc at gcc dot gnu dot org>
On Sat, 31 Mar 2001, Franz Sirl wrote:
> I'm currently tracking an excessive stack usage problem with DImode and
> inlined functions with gcc-2.95.3 on powerpc with the appended testcase. It
> turned out that the problem is still present in 3.0 and mainline.
>
> What I'm not sure about is whether to *first* scheduler pass (before reload)
> is allowed to destroy REG_LIBCALL/REG_RETVAL blocks? Are there any rules if
> so? The documentation is unclear in this respect.
It does happen, and so far we've modified the register allocators when this
has caused problems.
> It seems that the destructive scheduling of these blocks confuses reload and
> causes the excessive stack usage. In fact, if I add -fno-schedule-insns the
> stack usage is normal (just n DImode-sized stackslots remain for the inlined
> return values that we can't currently optimize away), 176 bytes. Otherwise
> the stackusage is between 560 and more than 3000 bytes. This is deadly for
> the limited kernel stack space in linux and was triggered by ReiserFS on PPC.
I'm not sure this has anything to do with REG_LIBCALL; it may simply be the
usual case of the first scheduling pass increasing register pressure too
much.
Bernd