http://llvm.org/bugs/show_bug.cgi?id=17195
Bug ID: 17195
Summary: ASM-based SIMD code -O2 performance and -g compilation
errors
Product: clang
Version: 3.3
Hardware: PC
OS: Linux
Status: NEW
Severity: normal
Priority: P
Component: C++
Assignee: [email protected]
Reporter: [email protected]
CC: [email protected], [email protected]
Classification: Unclassified
Created attachment 11182
--> http://llvm.org/bugs/attachment.cgi?id=11182&action=edit
test case described in bug report
Our C++ code embeds asm code for a calculation using SIMD instructions. There
are 2 problems demonstrated by the attached test case.
(1) With G++ I have to use -fno-dse for optimized code; without it the code
runs much more slowly than the equivalent non-SIMD version. With Clang++,
there is no such option, and the SIMD code runs more slowly than the non-SIMD
version.
The performance numbers look like this, running on Fedora 14 with a Xeon(R) CPU
X5680 @ 3.33GHz:
Clang++ SIMD version: 8.3 s.
Clang++ non-SIMD version: 5.8 s.
G++ SIMD version: 8.7 s.
G++ non-SIMD version: 11.2 s.
In the attached test case you can select the SIMD or non-SIMD version with a
#define macro in line 6.
Our Clang++ 3.3 was built with G++ 4.7.2. I uses these options for compiling
and linking: -march=core2 -msse4.1 -m64 -std=c++0x -fPIC -pthread
-Wno-logical-op-parentheses -Wno-shift-op-parentheses -O2 -g
I used G++ 4.5.1 with these options: -march=core2 -msse4.1 -m64 -mpc64
-std=c++0x -pedantic-errors -mieee-fp -fPIC -pthread -O2 -g
-fno-strict-aliasing -fno-tree-ccp -fno-dse
(2) Compiling with -g for a debug (non-optimized) version produces errors like
this for the SIMD version:
clang_test.cc:373:2: error: ran out of registers during register allocation
compute_factors_t (f0, f1, f2, f3, f0_1, f1_1, f2_1, f3_1,
^
clang_test.cc:197:10: note: expanded from macro 'compute_factors_t'
asm ("movapd %1, %%xmm0 \n\t" \
^
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