The v4 series of the gcc
compiler can automatically vectorize loops using the SIMD processor on some modern CPUs, such as the AMD Athlon or Intel Pentium/Core chips. How is this done?
The v4 series of the gcc compiler can automatically vectorize loops using the SIMD processor on some modern CPUs, such as the AMD Athlon or Intel Pentium/Core chips.
The GNU Compiler Collection, gcc, offers multiple ways to perform SIMD calculations.
Loop vectorization transforms procedural loops by assigning a processing unit to each pair of operands. Programs spend most of their time within such loops. Therefore, vectorization can significantly accelerate them, especially over large data sets.
Using -march=native enables all instruction subsets supported by the local machine (hence the result might not run on different machines). Using -mtune=native produces code optimized for the local machine under the constraints of the selected instruction set. A generic CPU with 64-bit extensions.
The original page offers details on getting gcc to automatically vectorize loops, including a few examples:
http://gcc.gnu.org/projects/tree-ssa/vectorization.html
While the examples are great, it turns out the syntax for calling those options with latest GCC seems to have changed a bit, see now:
In summary, the following options will work for x86 chips with SSE2, giving a log of loops that have been vectorized:
gcc -O2 -ftree-vectorize -msse2 -mfpmath=sse -ftree-vectorizer-verbose=5
Note that -msse is also a possibility, but it will only vectorize loops
using floats, not doubles or ints. (SSE2 is baseline for x86-64. For 32-bit code use -mfpmath=sse
as well. That's the default for 64-bit but not 32-bit.)
Modern versions of GCC enable -ftree-vectorize
at -O3
so just use that in GCC4.x and later:
gcc -O3 -msse2 -mfpmath=sse -ftree-vectorizer-verbose=5
(Clang enables auto-vectorization at -O2
. ICC defaults to optimization enabled + fast-math.)
Most of the following was written by Peter Cordes, who could have just written a new answer. Over time, as compilers change, options and compiler output will change. I am not entirely sure whether it is worth tracking it in great detail here. Comments? -- Author
To also use instruction set extensions supported by the hardware you're compiling on, and tune for it, use -march=native
.
Reduction loops (like sum of an array) will need OpenMP or -ffast-math
to treat FP math as associative and vectorize. Example on the Godbolt compiler explorer with -O3 -march=native -ffast-math
including a reduction (array sum) which is scalar without -ffast-math
. (Well, GCC8 and later do a SIMD load and then unpack it to scalar elements, which is pointless vs. simple unrolling. The loop bottlenecks on the latency of the one addss
dependency chain.)
Sometimes you don't need -ffast-math
, just -fno-math-errno
can help gcc inline math functions and vectorize something involving sqrt
and/or rint
/ nearbyint
.
Other useful options include -flto
(link-time optimization for cross-file inlining, constant propagation, etc) and / or profile-guided optimization with -fprofile-generate
/ test run(s) with realistic input(s) /-fprofile-use
. PGO enables loop unrolling for "hot" loops; in modern GCC that's off by default even at -O3.
There is a gimple (an Intermediate Representation of GCC) pass pass_vectorize
. This pass will enable auto-vectorization at gimple level.
For enabling autovectorization (GCC V4.4.0), we need to following steps:
UNITS_PER_SIMD_WORD
.<target>-modes.def
. This file has to reside in the directory where other files containing the machine descriptions are residing on. (As per the configuration script. If you can change the script you can place the file in whatever directory you want it to be in).The modes that are to be considered for vectorization as per target architecture. Like, 4 words will constitute a vector or eight half words will constitute a vector or two double-words will constitute a vector. The details of this needs to be mentioned in the <target>-modes.def
file. For example:
VECTOR_MODES (INT, 8); /* V8QI V4HI V2SI /
VECTOR_MODES (INT, 16); / V16QI V8HI V4SI V2DI /
VECTOR_MODES (FLOAT, 8); / V4HF V2SF */
Build the port. Vectorization can be enabled using the command line options -O2 -ftree-vectorize
.
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