https://gcc.gnu.org/bugzilla/show_bug.cgi?id=118570
Bug ID: 118570
Summary: -O2 much faster than -O3 for Romberg's method
Product: gcc
Version: 15.0
Status: UNCONFIRMED
Keywords: missed-optimization
Severity: normal
Priority: P3
Component: tree-optimization
Assignee: unassigned at gcc dot gnu.org
Reporter: sjames at gcc dot gnu.org
Target Milestone: ---
Created attachment 60215
--> https://gcc.gnu.org/bugzilla/attachment.cgi?id=60215&action=edit
rom3.c
This came up in #gcc on libera where trn reported an oddity, where -O2 is
significantly faster than -O3:
```
$ gcc -O2 rom3.c -o rom3 -lm && time ./rom3
rom3.c: In function ‘romberg’:
rom3.c:33:1: warning: old-style function definition [-Wold-style-definition]
33 | romberg(a, b, acc)
| ^~~~~~~
9558673.398323269560933
real 0m0.008s
user 0m0.002s
sys 0m0.006s
$ gcc -O3 rom3.c -o rom3 -lm && time ./rom3
rom3.c: In function ‘romberg’:
rom3.c:33:1: warning: old-style function definition [-Wold-style-definition]
33 | romberg(a, b, acc)
| ^~~~~~~
9558673.398323269560933
real 0m1.674s
user 0m1.658s
sys 0m0.004s
```
This is with recent trnuk but I see it on older releases too. -O3 -fno-ipa-cp
gives the same performance as -O2.
