[llvm-commits] PATCH: Switch StringMap to the new hashing infrastructure
Chandler Carruth
chandlerc at google.com
Mon Mar 5 02:17:20 PST 2012
Hello folks,
This is the oh-so-controversial cut-over. ;] I'm continuing to convert
other places which aren't quite so performance sensitive or (in most cases)
use significantly slower functions, but I wanted to get confirmation on
this one.
I've performed as careful analysis of the performance impact of this change
as I can. I've run the lex-only mode with before and after clang binaries
over very large source files. The runs were under the Linux 'perf' tool
which does hardware perf-event based instrumenting of the execution, and I
had it perform 40 runs of each command. I've run these with a forced
maximum kernel scheduling priority and affinity pinned to a single CPU.
This reduces the context switches to one (the CC1 invocation), results in
zero CPU migrations, and ensures that all measurements occur on the same
core of the same chip with a warm cache. The timings are (consequentially)
quite stable.
Here are the numbers for single-source GCC:
Performance counter stats for './bin/old_clang -fsyntax-only -Xclang
-Eonly ../tools/clang/INPUTS/gcc.c -w' (40 runs):
653.285653 task-clock # 0.998 CPUs utilized
( +- 0.19% )
1 context-switches # 0.000 M/sec
( +- 0.00% )
0 CPU-migrations # 0.000 M/sec
( +- 42.37% )
21,909 page-faults # 0.034 M/sec
( +- 0.00% )
1,656,298,378 cycles # 2.535 GHz
( +- 0.18% )
584,610,147 stalled-cycles-frontend # 35.30% frontend cycles
idle ( +- 0.52% )
368,002,172 stalled-cycles-backend # 22.22% backend cycles
idle ( +- 0.77% )
2,240,704,537 instructions # 1.35 insns per cycle
# 0.26 stalled cycles per
insn ( +- 0.02% )
494,832,319 branches # 757.452 M/sec
( +- 0.02% )
17,343,711 branch-misses # 3.50% of all branches
( +- 0.05% )
0.654568555 seconds time elapsed
( +- 0.19% )
Performance counter stats for './bin/new_clang -fsyntax-only -Xclang
-Eonly ../tools/clang/INPUTS/gcc.c -w' (40 runs):
652.817626 task-clock # 0.998 CPUs utilized
( +- 0.16% )
1 context-switches # 0.000 M/sec
( +- 0.00% )
0 CPU-migrations # 0.000 M/sec
( +- 69.80% )
21,913 page-faults # 0.034 M/sec
( +- 0.00% )
1,655,504,961 cycles # 2.536 GHz
( +- 0.16% )
579,918,671 stalled-cycles-frontend # 35.03% frontend cycles
idle ( +- 0.47% )
356,098,374 stalled-cycles-backend # 21.51% backend cycles
idle ( +- 0.72% )
2,240,570,707 instructions # 1.35 insns per cycle
# 0.26 stalled cycles per
insn ( +- 0.02% )
486,263,162 branches # 744.868 M/sec
( +- 0.02% )
16,554,052 branch-misses # 3.40% of all branches
( +- 0.05% )
0.654093465 seconds time elapsed
( +- 0.16% )
Here are the numbers for a single-source file with every source file in
Lex, Parse, and Sema from Clang included into it:
Performance counter stats for './bin/old_clang -x c++ -fsyntax-only
-Xclang -Eonly -I../include -Iinclude -I../tools/clang/include
-Itools/clang/include ../tools/clang/INPUTS/all-clang.cpp' (40 runs):
249.007880 task-clock # 0.998 CPUs utilized
( +- 0.27% )
1 context-switches # 0.000 M/sec
( +- 0.00% )
0 CPU-migrations # 0.000 M/sec
( +- 48.04% )
9,013 page-faults # 0.036 M/sec
( +- 0.00% )
630,166,134 cycles # 2.531 GHz
( +- 0.26% )
243,962,215 stalled-cycles-frontend # 38.71% frontend cycles
idle ( +- 0.70% )
152,285,530 stalled-cycles-backend # 24.17% backend cycles
idle ( +- 0.90% )
776,325,373 instructions # 1.23 insns per cycle
# 0.31 stalled cycles per
insn ( +- 0.03% )
180,715,948 branches # 725.744 M/sec
( +- 0.02% )
6,543,840 branch-misses # 3.62% of all branches
( +- 0.03% )
0.249614202 seconds time elapsed
( +- 0.27% )
Performance counter stats for './bin/new_clang -x c++ -fsyntax-only
-Xclang -Eonly -I../include -Iinclude -I../tools/clang/include
-Itools/clang/include ../tools/clang/INPUTS/all-clang.cpp' (40 runs):
247.344347 task-clock # 0.998 CPUs utilized
( +- 0.24% )
1 context-switches # 0.000 M/sec
( +- 0.00% )
0 CPU-migrations # 0.000 M/sec
( +- 0.00% )
9,018 page-faults # 0.036 M/sec
( +- 0.00% )
626,293,160 cycles # 2.532 GHz
( +- 0.23% )
240,474,650 stalled-cycles-frontend # 38.40% frontend cycles
idle ( +- 0.60% )
147,531,931 stalled-cycles-backend # 23.56% backend cycles
idle ( +- 0.84% )
767,828,567 instructions # 1.23 insns per cycle
# 0.31 stalled cycles per
insn ( +- 0.03% )
173,028,132 branches # 699.544 M/sec
( +- 0.02% )
6,031,206 branch-misses # 3.49% of all branches
( +- 0.04% )
0.247948747 seconds time elapsed
( +- 0.24% )
The measured changes in performance are all down into the noise floor,
despite everything I did to lower that. From what I can tell, this change
has no observable performance impact, and reduces the number of hashing
functions in use. Good to commit?
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