ralloc UNSPEC vs UNSPEC_VOLATILE

[Hariharan Sandanagobalane]

Hello,
Picochip has communication instructions that allow one array element to 
pass data to another. There are 3 such instructions PUT/GET/TSTPORT. 
Currently, all three of these use UNSPEC_VOLATILE side-effect 
expressions to make sure they don't get reordered with respect to each 
other.

There were quite a few testcases where virtual registers assigned to 
communication instructions were being spilled, even in functions where 
register pressure is not very high. I looked at the data flow 
information and it looks like the virtual registers involved in these 
UNSPEC_VOLATILE operations are marked to interfere with every live range.

To avoid unnecessary register spill due to UNSPEC_VOLATILE rtl 
expressions, i converted these patterns to use UNSPEC expressions. But, 
the live ranges of virtual registers involved here still show the same 
characteristics as above. Is this right? Why does the live range of a 
virtual register involved in UNSPEC operation be marked to interfere 
with live ranges of all the other virtual registers?

I tried looking into the code, but i am not sure how UNSPEC patterns are 
handled.

Please let me know if i am missing something here.

Cheers
Hari

PS: I attach a concrete testcase, with dumps of sched and ira. In the 
mainline case, it is virtual register 36. In unspec-patch case, it is 
virtual register 37. I attach the unspec patch to picochip port, which 
you can apply to mainline if you would like to test.
-------------- next part --------------
A non-text attachment was scrubbed...
Name: mainline.tgz
Type: application/x-compressed-tar
Size: 118525 bytes
Desc: not available
URL: <https://gcc.gnu.org/pipermail/gcc/attachments/20101018/4ef77ae2/attachment.bin>
-------------- next part --------------
A non-text attachment was scrubbed...
Name: unspec_patched.tgz
Type: application/x-compressed-tar
Size: 119032 bytes
Desc: not available
URL: <https://gcc.gnu.org/pipermail/gcc/attachments/20101018/4ef77ae2/attachment-0001.bin>
-------------- next part --------------
An embedded and charset-unspecified text was scrubbed...
Name: unspec_patch
URL: <https://gcc.gnu.org/pipermail/gcc/attachments/20101018/4ef77ae2/attachment.ksh>