[Bug middle-end/51182] [ipa-iterations] running multiple passes of early IPA on a file produces different code when it shouldn't

["matt at use dot net" <gcc-bugzilla@gcc.gnu.org>]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=51182

--- Comment #4 from Matt Hargett <matt at use dot net> 2011-11-18 01:43:32 UTC ---
Ah, okay. I read in your email you were looking for evidence of bugs, and the
behaviour looked fishy to me. Regardless, here is a performance improvement
that perhaps should be gotten within one iteration.

Attached is the combined.i from pmccabe, which can be compiled and linked
directly to be an executable (on a Debian/Ubuntu-ish amd64 system, anyway).

Using -O3 (or -Ofast), two iterations produces a binary that performs better
than just one iteration. Performance was measured at the macro level, based on
timings when run against tens of thousands of files while in single-user mode
on a ramdisk. In addition, performance at the micro level was measured by
looking at cache misses and branch misprediction rates using callgrind (a tool
within valgrind), with output below. The second iteration reduces the I1 miss
rate, as well as the misprediction rate. (Multiple iterations of -O2 is more of
a mixed bag at the micro level, for some reason, and appears to have no
macro-level performance impact.)


matt@matt-desktop:~/src/pmccabe-2.7$ valgrind --tool=callgrind --branch-sim=yes
--cache-sim=yes ./pmccabe.o3i1.loop.whopr *.c test0[012][0123456]

==4119== 
==4119== Events    : Ir Dr Dw I1mr D1mr D1mw ILmr DLmr DLmw Bc Bcm Bi Bim
==4119== Collected : 10312284 2549768 1398563 3869 3209 1417 1285 2045 990
2534056 74514 208896 8052
==4119== 
==4119== I   refs:      10,312,284
==4119== I1  misses:         3,869
==4119== LLi misses:         1,285
==4119== I1  miss rate:        0.3%
==4119== LLi miss rate:        0.1%
==4119== 
==4119== D   refs:       3,948,331  (2,549,768 rd + 1,398,563 wr)
==4119== D1  misses:         4,626  (    3,209 rd +     1,417 wr)
==4119== LLd misses:         3,035  (    2,045 rd +       990 wr)
==4119== D1  miss rate:        0.1% (      0.1%   +       0.1%  )
==4119== LLd miss rate:        0.0% (      0.0%   +       0.0%  )
==4119== 
==4119== LL refs:            8,495  (    7,078 rd +     1,417 wr)
==4119== LL misses:          4,320  (    3,330 rd +       990 wr)
==4119== LL miss rate:         0.0% (      0.0%   +       0.0%  )
==4119== 
==4119== Branches:       2,742,952  (2,534,056 cond +   208,896 ind)
==4119== Mispredicts:       82,566  (   74,514 cond +     8,052 ind)
==4119== Mispred rate:         3.0% (      2.9%     +       3.8%   )


matt@matt-desktop:~/src/pmccabe-2.7$ valgrind --tool=callgrind --branch-sim=yes
--cache-sim=yes ./pmccabe.o3i2.loop.whopr *.c test0[012][0123456]

==4122== 
==4122== Events    : Ir Dr Dw I1mr D1mr D1mw ILmr DLmr DLmw Bc Bcm Bi Bim
==4122== Collected : 10312147 2549768 1398563 3054 3209 1416 1286 2049 989
2534056 74071 208896 7618
==4122== 
==4122== I   refs:      10,312,147
==4122== I1  misses:         3,054
==4122== LLi misses:         1,286
==4122== I1  miss rate:        0.2%
==4122== LLi miss rate:        0.1%
==4122== 
==4122== D   refs:       3,948,331  (2,549,768 rd + 1,398,563 wr)
==4122== D1  misses:         4,625  (    3,209 rd +     1,416 wr)
==4122== LLd misses:         3,038  (    2,049 rd +       989 wr)
==4122== D1  miss rate:        0.1% (      0.1%   +       0.1%  )
==4122== LLd miss rate:        0.0% (      0.0%   +       0.0%  )
==4122== 
==4122== LL refs:            7,679  (    6,263 rd +     1,416 wr)
==4122== LL misses:          4,324  (    3,335 rd +       989 wr)
==4122== LL miss rate:         0.0% (      0.0%   +       0.0%  )
==4122== 
==4122== Branches:       2,742,952  (2,534,056 cond +   208,896 ind)
==4122== Mispredicts:       81,689  (   74,071 cond +     7,618 ind)
==4122== Mispred rate:         2.9% (      2.9%     +       3.6%   )