https://gcc.gnu.org/bugzilla/show_bug.cgi?id=21485
Richard Biener <rguenth at gcc dot gnu.org> changed:
What |Removed |Added
----------------------------------------------------------------------------
Last reconfirmed|2018-02-01 00:00:00 |2025-2-10
--- Comment #78 from Richard Biener <rguenth at gcc dot gnu.org> ---
(In reply to Thomas Koenig from comment #77)
> Just wondering... has this been fixed in the meantime?
With GCC 3.4.6 I get a score of 7074.5 while GCC 15 gets 3304.7, with
-fno-tree-pre the score is 7001.
So no.
Note even GCC 4.0.4 gets just 3342.7, with -fno-tree-pre its 5449.3 there.
These are now measurements on the Zen4 uarch.
Profile from 3.4.6:
│150:┌─→mov %rdx,(%rdi,%rax,8)
▒
215 │ │ mov %rcx,(%rdi,%r9,8)
▒
473 │ │ mov %rax,%r9
▒
11 │ │ lea (%r9,%r9,1),%rdx
▒
291 │ │ cmp %r8,%rdx
▒
│ │↓ ja 197
▒
│164:│ cmp %r8,%rdx
▒
110 │ │ mov %rdx,%rax
▒
225 │ │↓ jae 17d
▒
│ │ mov 0x8(%rdi,%rdx,8),%rbx
▒
3603 │ │ cmp %rbx,(%rdi,%rdx,8)
▒
1924 │ │ lea 0x1(%rdx),%rax
▒
364 │ │ cmovge %rdx,%rax
▒
1554 │17d:│ mov (%rdi,%r9,8),%rdx
▒
1681 │ │ mov (%rdi,%rax,8),%rcx
◆
7092 │ ├──cmp %rcx,%rdx
▒
1130 │ └──jl 150
and from GCC 15:
│240:┌─→mov %rbp,(%r12)
▒
799 │ │ mov %r8,(%rax)
▒
77 │ │ lea (%rdx,%rdx,1),%rax
▒
113 │ │ cmp %rax,%rdi
▒
5 │ │↓ jb 2b0
▒
│250:│ mov %rdx,%rbp
▒
92 │253:│ cmp %rdi,%rax
▒
3 │ │↓ jae 271
▒
│ │ lea 0x1(%rax),%rdx
▒
77 │ │ mov %rbp,%r15
▒
125 │ │ lea (%rbx,%rdx,8),%r12
▒
265 │ │ shl $0x4,%r15
▒
448 │ │ mov (%r12),%r8
▒
3436 │ │ cmp %r8,(%rbx,%r15,1)
▒
770 │ │↓ jl 27c
▒
│271:│ lea (%rbx,%rax,8),%r12
▒
3344 │ │ mov %rax,%rdx
◆
11 │ │ mov (%r12),%r8
▒
923 │27c:│ lea (%rbx,%rbp,8),%rax
▒
3927 │ │ mov (%rax),%rbp
▒
2308 │ ├──cmp %r8,%rbp
▒
51 │ └──jl 240
so it looks like the if-conversion done by GCC 3.4 is missing here and the
PRE of the (%r12) load to %r8 is inhibiting it.
