On Mon, Jul 13, 2015 at 12:17 AM, Prathamesh Kulkarni
<prathamesh.kulka...@linaro.org> wrote:
> - for -O2 temps introduced by peephole2 due to define_peephole2
> pattern in thumb2.md:1540
> http://pastebin.com/3rEF8Te4
> So this intentionally transforms rtx from
> zeroextractsi_compare0_scratch to rtx from shiftsi3_compare0_scratch.
> Why is it beneficial to do this transform ?
I used svn blame to find the revision where the pattern was added,
then svn log to get the changelog entry and date from that revision,
and searching the gcc-patches mailing list on that date I found this
https://gcc.gnu.org/ml/gcc-patches/2010-06/msg02518.html
It doesn't explain much, but refers to an earlier change, so I did a
google search for thumb2_tlobits_cbranch to find when this pattern was
added, and that gives me
https://gcc.gnu.org/ml/gcc-patches/2010-05/msg00003.html
which refers to bug 42879 which has the explanation we want
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=42879
tst is a 32-bit thumb2 instruction, but lsls is a 16-bit instruction,
so it is a minor size optimization.
Jim
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