PATCH: Optimize IA64 brl to br

Jim Wilson wilson@specifixinc.com
Fri Jan 30 06:23:00 GMT 2004


On Thu, 2004-01-29 at 19:09, Richard Henderson wrote:
> Is MBB really the best bundle to use for this?  I seem to recall
> that forcing a split issue.  I'd think MIB or MMB would be better.

Good point.  I didn't check that.

See section 3.3.2 "Dispersal Rules" of the Itanium2 Processor Reference
Manual, in particular see table 3-4 on page 16 which shows that MBB
always splits issue.  However, the same section points out that MIB,
MMB, and MFB always split issue unless the B insn is a nop.b or a brp
(branch predict).  So it does not appear to make any difference.

The table 3-4 has some typos incidentally.  I have the June 2002 version
of the document.  MBB is mentioned twice, both in the row and column
indices.  The second one is supposed to be MMB in both cases.  Also, the
MMB and MFB row entries are missing the 1 superscript that is present on
the MIB row entry, to indicate that dual-issue only occurs if the B is a
nop.b or brp.

I don't see anything that would suggest any of these is better than the
other.
-- 
Jim Wilson, GNU Tools Support, http://www.SpecifixInc.com



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