Phoronix numbers for clang-omp compiler

Hi All,

Some of you probably saw performance benchmarking of OpenMP-enabled clang compiler published on Phoronix web-site: http://www.phoronix.com/scan.php?page=article&item=llvm_clang_openmp&num=1

As you can see, clang-omp is behind gcc in most of the benchmarks. The reason is quite simple: no -O options supplied to both compilers, so gcc assumed -O2 by default, while clang assumed -O0. No surprise gcc got ahead!

We added -O2 to clang and re-measured, and results changed quite significantly. My Intel colleague, Alexey Bataev, informed Michael Larabel on this omission.

Yours,
Andrey

Andrey,
FSF gcc is not exactly defaulting to -O2 but certainly is higher than the default optimizations on clang….

% touch t.cc
% g+±fsf-4.9 -fverbose-asm t.cc -S
% more t.s

GNU C++ (GCC) version 4.9.0 (x86_64-apple-darwin11.4.2)

compiled by GNU C version 4.9.0, GMP version 6.0.0, MPFR version 3.1.2, MPC version 1.0.2

GGC heuristics: --param ggc-min-expand=100 --param ggc-min-heapsize=131072

options passed: -D__DYNAMIC__ t.cc -fPIC -mmacosx-version-min=10.7.4

-mtune=core2 -fverbose-asm

options enabled: -Wnonportable-cfstrings -fPIC

-faggressive-loop-optimizations -fasynchronous-unwind-tables

-fauto-inc-dec -fcommon -fdelete-null-pointer-checks -fearly-inlining

-feliminate-unused-debug-types -fexceptions -ffunction-cse -fgcse-lm

-fgnu-unique -fident -finline-atomics -fira-hoist-pressure

-fira-share-save-slots -fira-share-spill-slots -fivopts

-fkeep-static-consts -fleading-underscore -fmath-errno

-fmerge-debug-strings -fnext-runtime -fobjc-abi-version= -fpeephole

-fprefetch-loop-arrays -freg-struct-return

-fsched-critical-path-heuristic -fsched-dep-count-heuristic

-fsched-group-heuristic -fsched-interblock -fsched-last-insn-heuristic

-fsched-rank-heuristic -fsched-spec -fsched-spec-insn-heuristic

-fsched-stalled-insns-dep -fshow-column -fsigned-zeros

-fsplit-ivs-in-unroller -fstrict-volatile-bitfields -fsync-libcalls

-ftrapping-math -ftree-coalesce-vars -ftree-cselim -ftree-forwprop

-ftree-loop-if-convert -ftree-loop-im -ftree-loop-ivcanon

-ftree-loop-optimize -ftree-parallelize-loops= -ftree-phiprop

-ftree-reassoc -ftree-scev-cprop -funit-at-a-time -funwind-tables

-fverbose-asm -fzero-initialized-in-bss -gstrict-dwarf

-m128bit-long-double -m64 -m80387 -malign-stringops -matt-stubs

-mconstant-cfstrings -mfancy-math-387 -mfp-ret-in-387 -mfxsr -mieee-fp

-mlong-double-80 -mmmx -mno-sse4 -mpush-args -mred-zone -msse -msse2

-msse3

.constructor
.destructor
.align 1
.subsections_via_symbols

% g+±fsf-4.9 -fverbose-asm -O2 t.cc -S
% more t.s

GNU C++ (GCC) version 4.9.0 (x86_64-apple-darwin11.4.2)

compiled by GNU C version 4.9.0, GMP version 6.0.0, MPFR version 3.1.2, MPC version 1.0.2

GGC heuristics: --param ggc-min-expand=100 --param ggc-min-heapsize=131072

options passed: -D__DYNAMIC__ t.cc -fPIC -mmacosx-version-min=10.7.4

-mtune=core2 -O2 -fverbose-asm

options enabled: -Wnonportable-cfstrings -fPIC

-faggressive-loop-optimizations -fasynchronous-unwind-tables

-fauto-inc-dec -fbranch-count-reg -fcaller-saves

-fcombine-stack-adjustments -fcommon -fcompare-elim -fcprop-registers

-fcrossjumping -fcse-follow-jumps -fdefer-pop

-fdelete-null-pointer-checks -fdevirtualize -fdevirtualize-speculatively

-fearly-inlining -feliminate-unused-debug-types -fexceptions

-fexpensive-optimizations -fforward-propagate -ffunction-cse -fgcse

-fgcse-lm -fgnu-unique -fguess-branch-probability -fhoist-adjacent-loads

-fident -fif-conversion -fif-conversion2 -findirect-inlining -finline

-finline-atomics -finline-functions-called-once -finline-small-functions

-fipa-cp -fipa-profile -fipa-pure-const -fipa-reference -fipa-sra

-fira-hoist-pressure -fira-share-save-slots -fira-share-spill-slots

-fisolate-erroneous-paths-dereference -fivopts -fkeep-static-consts

-fleading-underscore -fmath-errno -fmerge-constants -fmerge-debug-strings

-fmove-loop-invariants -fnext-runtime -fobjc-abi-version=

-fomit-frame-pointer -foptimize-sibling-calls -foptimize-strlen

-fpartial-inlining -fpeephole -fpeephole2 -fprefetch-loop-arrays -free

-freg-struct-return -freorder-blocks -freorder-blocks-and-partition

-freorder-functions -frerun-cse-after-loop

-fsched-critical-path-heuristic -fsched-dep-count-heuristic

-fsched-group-heuristic -fsched-interblock -fsched-last-insn-heuristic

-fsched-rank-heuristic -fsched-spec -fsched-spec-insn-heuristic

-fsched-stalled-insns-dep -fschedule-insns2 -fshow-column -fshrink-wrap

-fsigned-zeros -fsplit-ivs-in-unroller -fsplit-wide-types

-fstrict-aliasing -fstrict-overflow -fstrict-volatile-bitfields

-fsync-libcalls -fthread-jumps -ftoplevel-reorder -ftrapping-math

-ftree-bit-ccp -ftree-builtin-call-dce -ftree-ccp -ftree-ch

-ftree-coalesce-vars -ftree-copy-prop -ftree-copyrename -ftree-cselim

-ftree-dce -ftree-dominator-opts -ftree-dse -ftree-forwprop -ftree-fre

-ftree-loop-if-convert -ftree-loop-im -ftree-loop-ivcanon

-ftree-loop-optimize -ftree-parallelize-loops= -ftree-phiprop -ftree-pre

-ftree-pta -ftree-reassoc -ftree-scev-cprop -ftree-sink -ftree-slsr

-ftree-sra -ftree-switch-conversion -ftree-tail-merge -ftree-ter

-ftree-vrp -funit-at-a-time -funwind-tables -fverbose-asm

-fzero-initialized-in-bss -gstrict-dwarf -m128bit-long-double -m64

-m80387 -malign-stringops -matt-stubs -mconstant-cfstrings

-mfancy-math-387 -mfp-ret-in-387 -mfxsr -mieee-fp -mlong-double-80 -mmmx

-mno-sse4 -mpush-args -mred-zone -msse -msse2 -msse3 -mvzeroupper

.constructor
.destructor
.align 1
.subsections_via_symbols

You might consider filing an enhancement request for clang 3.5 to have the default behavior without optimization flags bumped up to -O1.
Jack