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Re: [PATCH] support to flip modes in sh mode-switching
- From: Uros Bizjak <ubizjak at gmail dot com>
- To: Christian BRUEL <christian dot bruel at st dot com>
- Cc: 'GCC Patches' <gcc-patches at gcc dot gnu dot org>, amylaar at gcc dot gnu dot org
- Date: Sat, 23 Dec 2006 12:33:39 +0100
- Subject: Re: [PATCH] support to flip modes in sh mode-switching
- References: <000501c72609$f1f588b0$0b00fb0a@gnb.st.com>
Christian BRUEL wrote:
Thanks for the pointers. From what I see from #19161, the mode-switching
pass would need to be connected to the register class selection ? That's
more that what it does but that would be interesting to see how it fits. For
No, for SH, you don't have to select different register classes. This is
specific to x87->mmx switch, as no values should live in any of
registers during the switch. This was achieved by modelling emms to
"use" all x87 registers (and similar for efpu insn and MMX register),
and this info was propagated over edges to previous blocks. In some
cases involving two or more incoming edges, both register sets can
become "used" on a BB entry, and no further insn could use neither MMX
nor x87 registers.
I was hoping that by your mode-flipping approach, exclusivity of modes
can be obtained. This is IMO the same problem as with SH-4a mode
flipping patch. Consider the case, where one incoming edge is set to
DFmode, while the other is set to SFmode, and somewhere in the BB your
insn wants i.e. DFmode. With mode-setting, the situation is clear; you
force mode to DFmode before insn. But for mode-flipping approach, do you
want to flip mode or not?
Uros.