[patch] Improve stack checking on PowerPC

[Eric Botcazou]

Stack checking now works reasonably well on the mainline, however it is a 
little inefficient on the PowerPC.  The reason is that it is implemented (on 
OSes with MM) by means of probes, i.e. by storing 0 at specified addresses.

Now the PowerPC doesn't feature a register hardwired to 0 like the SPARC so 0 
needs to be loaded each time, which is useless since the value being stored 
doesn't matter, only the store does.

Hence the attached patch, tested on powerpc-linux-gnu, OK for mainline?


2009-12-13  Eric Botcazou  <ebotcazou@adacore.com>

	* config/rs6000/rs6000.md (UNSPEC_PROBE_STACK): New constant.
	(probe_stack): New insn.


-- 
Eric Botcazou
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