https://gcc.gnu.org/bugzilla/show_bug.cgi?id=98868
--- Comment #3 from Martin Liška <marxin at gcc dot gnu.org> ---
It's likely about a small loop alignment:
# Overhead Command Shared Object Symbol
# ........ ....... .................... ....................................
#
78.19% a.out a.out [.] matsim_
17.00% a.out a.out [.] evlrnf_
matsim_ hot place (with --show-total-period)
SLOW:
8653282 : 4017cb: imul $0x3243f6ad,%esi,%esi
: genuni():
: genuni = us231 * real (jsee)
726254541 : 4017d1: vxorps %xmm0,%xmm0,%xmm0
: jsee = jsee * jmul + jadd
0 : 4017d5: add $0x1b0cb175,%esi
: genuni = us231 * real (jsee)
105853662 : 4017db: vcvtsi2ss %esi,%xmm0,%xmm0
273371557 : 4017df: vmulss %xmm1,%xmm0,%xmm0
: gentrs_():
: do icls = icls1, ncls
454049783 : 4017e3: cmp $0xffffffff,%edi
2165881 : 4017e6: je 401970 <matsim_+0x470>
0 : 4017ec: cmp $0x1,%edi
1081799 : 4017ef: jne 4017cb <matsim_+0x2cb>
2155914 : 4017f1: mov %r9,%rdx
4307088 : 4017f4: mov %r8d,%ecx
0 : 4017f7: jmp 401811 <matsim_+0x311>
0 : 4017f9: nopl 0x0(%rax)
8624612913 : 401800: inc %ecx
42153493 : 401802: add $0x400,%rdx
484044717 : 401809: cmp $0x101,%ecx
38933067 : 40180f: je 4017cb <matsim_+0x2cb>
FAST:
45442445 : 4017c9: imul $0x3243f6ad,%edx,%edx
: genuni():
: genuni = us231 * real (jsee)
1076892 : 4017cf: vxorps %xmm0,%xmm0,%xmm0
: jsee = jsee * jmul + jadd
3245642 : 4017d3: add $0x1b0cb175,%edx
: jsee = ibits(jsee, 0, 31) !
Replacement
1083699 : 4017d9: and $0x7fffffff,%edx
: genuni = us231 * real (jsee)
0 : 4017df: vcvtsi2ss %edx,%xmm0,%xmm0
76652291 : 4017e3: vmulss %xmm1,%xmm0,%xmm0
: gentrs_():
: do icls = icls1, ncls
166631920 : 4017e7: cmp $0xffffffff,%edi
3251886 : 4017ea: je 401970 <matsim_+0x470>
0 : 4017f0: cmp $0x1,%edi
0 : 4017f3: jne 4017c9 <matsim_+0x2c9>
0 : 4017f5: mov %r9,%rcx
0 : 4017f8: mov %r8d,%esi
1083364 : 4017fb: jmp 401811 <matsim_+0x311>
0 : 4017fd: nopl (%rax)
1099920836 : 401800: inc %esi
209587136 : 401802: add $0x400,%rcx
100391619 : 401809: cmp $0x101,%esi
69184337 : 40180f: je 4017c9 <matsim_+0x2c9>
For some reason the hottest "inc" instruction has in fast version ~10x smaller
number of cycles.
The instruction takes 20% of cycles in the slow version.
