https://llvm.org/bugs/show_bug.cgi?id=25108
Bug ID: 25108
Summary: 30% performance deficiency vs GCC because of excessive
vectorization on -march=slm
Product: clang
Version: trunk
Hardware: PC
OS: Linux
Status: NEW
Severity: normal
Priority: P
Component: LLVM Codegen
Assignee: unassignedclangb...@nondot.org
Reporter: egor.koche...@intel.com
CC: llvm-bugs@lists.llvm.org
Classification: Unclassified
Created attachment 15033
--> https://llvm.org/bugs/attachment.cgi?id=15033&action=edit
c++ reproducer, Makefile, two disassembled functions.
Here is the C++ source that gets vectorized by clang and is not vectorized by
gcc, it is the function 'compute' of the attached cpp file.
template <typename T>
void compute (T* v) {
for (int i = 0; i < 10000; ++i) {
// fw pass
for (int i = 0; i < 6; ++i) {
v [i] += v [i + 1];
v [i] *= v [i + 2];
}
//back pass
for (int i = 7; i > 1; --i) {
v [i] += v [i - 1];
v [i] *= v [i - 2];
}
}
}
The vectorized version happens to work slower, when compiled both for float and
for double numbers, on a Silvermont Intel machine. The test run timings are as
follows:
clang:
float time: 0.000043 ms
double time: 0.000059 ms
gcc:
float time: 0.000029 ms
double time: 0.000043 ms
The source was compiled with -Ofast -m64 -g -march=slm -mfpmath=sse -fPIE. 32
bit build shows the same result.
This difference reproduces a real one which is measured in seconds in one of
the programs I deal with.
It seems, that vectorization costs in clang are not set up correctly for
-march=slm. When compiling by clang with -fno-slp-vectorize, the resulting
performance matches gcc.
Compilers under consideration are: gcc 6.0 trunk vs clang 3.8 trunk.
The attachment is the archive with the C++ reproducer source, Makefile and
assemblies for float version got from the two compilers.
To run the reproducer using the Makefile attached, call `make clean run; make
clean run COMPILER=gcc` after fixing compiler paths in the Makefile.
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