48cab67447
Optimize `IntRange::from_pat`, then shrink `ParamEnv` Resolves #77058. r? `@Mark-Simulacrum` cc `@vandenheuvel` Looking at the output of `perf report` for #76244, the hot instructions seemed to be around the call to `pat_constructor` in `IntRange::from_pat`. I carried out an obvious optimization, but it actually made the instruction count higher (see #77075). However, it seems to have mitigated whatever was causing the pipeline stalls, so when combined with #76244, it's a net win. As you can see below, the regression in #76244 seems to have originated from something measured by `stalled-cycles-backend`. I'll try to collect some finer-grained stats to see if I can isolate it. I wish I had a better idea of what was going on here. I'd like to prevent the regression from reappearing in the future due to small changes in unrelated code. <details> <summary>Current `master`:</summary> ``` Performance counter stats for 'cargo +baseline-stage1 check': 2,275.67 msec task-clock:u # 0.998 CPUs utilized 0 context-switches:u # 0.000 K/sec 0 cpu-migrations:u # 0.000 K/sec 49,826 page-faults:u # 0.022 M/sec 5,117,221,678 cycles:u # 2.249 GHz 299,655,943 stalled-cycles-frontend:u # 5.86% frontend cycles idle 2,284,213,395 stalled-cycles-backend:u # 44.64% backend cycles idle 8,051,871,959 instructions:u # 1.57 insn per cycle # 0.28 stalled cycles per insn 1,359,589,402 branches:u # 597.447 M/sec 7,359,347 branch-misses:u # 0.54% of all branches 2.281030026 seconds time elapsed 2.108197000 seconds user 0.164183000 seconds sys ``` </details> <details> <summary>Shrink `ParamEnv` without changing `IntRange::from_pat`:</summary> ``` Performance counter stats for 'cargo +perf-stage1 check': 2,751.79 msec task-clock:u # 0.996 CPUs utilized 0 context-switches:u # 0.000 K/sec 0 cpu-migrations:u # 0.000 K/sec 50,103 page-faults:u # 0.018 M/sec 6,260,590,019 cycles:u # 2.275 GHz 317,355,920 stalled-cycles-frontend:u # 5.07% frontend cycles idle 3,397,743,582 stalled-cycles-backend:u # 54.27% backend cycles idle 8,276,224,367 instructions:u # 1.32 insn per cycle # 0.41 stalled cycles per insn 1,370,453,386 branches:u # 498.023 M/sec 7,281,031 branch-misses:u # 0.53% of all branches 2.763265838 seconds time elapsed 2.544578000 seconds user 0.204548000 seconds sys ``` </details> <details> <summary>Shrink `ParamEnv` and change `IntRange::from_pat`: </summary> ``` Performance counter stats for 'cargo +perf-stage1 check': 2,295.57 msec task-clock:u # 0.996 CPUs utilized 0 context-switches:u # 0.000 K/sec 0 cpu-migrations:u # 0.000 K/sec 49,959 page-faults:u # 0.022 M/sec 5,151,407,066 cycles:u # 2.244 GHz 324,517,829 stalled-cycles-frontend:u # 6.30% frontend cycles idle 2,301,671,001 stalled-cycles-backend:u # 44.68% backend cycles idle 8,130,868,329 instructions:u # 1.58 insn per cycle # 0.28 stalled cycles per insn 1,356,618,512 branches:u # 590.972 M/sec 7,323,800 branch-misses:u # 0.54% of all branches 2.304509653 seconds time elapsed 2.128090000 seconds user 0.163909000 seconds sys ``` </details>
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