Here are some up-to-date albeit narrow findings of mine with GCC 4.7.2
and Clang 3.2 for C++.
UPDATE: GCC 4.8.1 v clang 3.3 comparison appended below.
UPDATE: GCC 4.8.2 v clang 3.4 comparison is appended to that.
I maintain an OSS tool that is built for Linux with both GCC and Clang,
and with Microsoft's compiler for Windows. The tool, coan, is a preprocessor
and analyser of C/C++ source files and codelines of such: its
computational profile majors on recursive-descent parsing and file-handling.
The development branch (to which these results pertain)
comprises at present around 11K LOC in about 90 files. It is coded,
now, in C++ that is rich in polymorphism and templates and but is still
mired in many patches by its not-so-distant past in hacked-together C.
Move semantics are not expressly exploited. It is single-threaded. I
have devoted no serious effort to optimizing it, while the "architecture"
remains so largely ToDo.
I employed Clang prior to 3.2 only as an experimental compiler
because, despite its superior compilation speed and diagnostics, its
C++11 standard support lagged the contemporary GCC version in the
respects exercised by coan. With 3.2, this gap has been closed.
My Linux test harness for current coan development processes roughly
70K sources files in a mixture of one-file parser test-cases, stress
tests consuming 1000s of files and scenario tests consuming < 1K files.
As well as reporting the test results, the harness accumulates and
displays the totals of files consumed and the run time consumed in coan
(it just passes each coan command line to the Linux time command and
captures and adds up the reported numbers). The timings are flattered
by the fact that any number of tests which take 0 measurable time will
all add up to 0, but the contribution of such tests is negligible. The
timing stats are displayed at the end of make check like this:
coan_test_timer: info: coan processed 70844 input_files.
coan_test_timer: info: run time in coan: 16.4 secs.
coan_test_timer: info: Average processing time per input file: 0.000231 secs.
I compared the test harness performance as between GCC 4.7.2 and
Clang 3.2, all things being equal except the compilers. As of Clang 3.2,
I no longer require any preprocessor differentiation between code
tracts that GCC will compile and Clang alternatives. I built to the
same C++ library (GCC's) in each case and ran all the comparisons
consecutively in the same terminal session.
The default optimization level for my release build is -O2. I also
successfully tested builds at -O3. I tested each configuration 3
times back-to-back and averaged the 3 outcomes, with the following
results. The number in a data-cell is the average number of
microseconds consumed by the coan executable to process each of
the ~70K input files (read, parse and write output and diagnostics).
| -O2 | -O3 |O2/O3|
----------|-----|-----|-----|
GCC-4.7.2 | 231 | 237 |0.97 |
----------|-----|-----|-----|
Clang-3.2 | 234 | 186 |1.25 |
----------|-----|-----|------
GCC/Clang |0.99 | 1.27|
Any particular application is very likely to have traits that play
unfairly to a compiler's strengths or weaknesses. Rigorous benchmarking
employs diverse applications. With that well in mind, the noteworthy
features of these data are:
- -O3 optimization was marginally detrimental to GCC
- -O3 optimization was importantly beneficial to Clang
- At -O2 optimization, GCC was faster than Clang by just a whisker
- At -O3 optimization, Clang was importantly faster than GCC.
A further interesting comparison of the two compilers emerged by accident
shortly after those findings. Coan liberally employs smart pointers and
one such is heavily exercised in the file handling. This particular
smart-pointer type had been typedef'd in prior releases for the sake of
compiler-differentiation, to be an std::unique_ptr<X> if the
configured compiler had sufficiently mature support for its usage as
that, and otherwise an std::shared_ptr<X>. The bias to std::unique_ptr was
foolish, since these pointers were in fact transferred around,
but std::unique_ptr looked like the fitter option for replacing
std::auto_ptr at a point when the C++11 variants were novel to me.
In the course of experimental builds to gauge Clang 3.2's continued need
for this and similar differentiation, I inadvertently built
std::shared_ptr<X> when I had intended to build std::unique_ptr<X>,
and was surprised to observe that the resulting executable, with default -O2
optimization, was the fastest I had seen, sometimes achieving 184
msecs. per input file. With this one change to the source code,
the corresponding results were these;
| -O2 | -O3 |O2/O3|
----------|-----|-----|-----|
GCC-4.7.2 | 234 | 234 |1.00 |
----------|-----|-----|-----|
Clang-3.2 | 188 | 187 |1.00 |
----------|-----|-----|------
GCC/Clang |1.24 |1.25 |
The points of note here are:
- Neither compiler now benefits at all from -O3 optimization.
- Clang beats GCC just as importantly at each level of optimization.
- GCC's performance is only marginally affected by the smart-pointer type
change.
- Clang's -O2 performance is importantly affected by the smart-pointer type
change.
Before and after the smart-pointer type change, Clang is able to build a
substantially faster coan executable at -O3 optimisation, and it can
build an equally faster executable at -O2 and -O3 when that
pointer-type is the best one - std::shared_ptr<X> - for the job.
An obvious question that I am not competent to comment upon is why
Clang should be able to find a 25% -O2 speed-up in my application when
a heavily used smart-pointer-type is changed from unique to shared,
while GCC is indifferent to the same change. Nor do I know whether I should
cheer or boo the discovery that Clang's -O2 optimization harbours
such huge sensitivity to the wisdom of my smart-pointer choices.
UPDATE: GCC 4.8.1 v clang 3.3
The corresponding results now are:
| -O2 | -O3 |O2/O3|
----------|-----|-----|-----|
GCC-4.8.1 | 442 | 443 |1.00 |
----------|-----|-----|-----|
Clang-3.3 | 374 | 370 |1.01 |
----------|-----|-----|------
GCC/Clang |1.18 |1.20 |
The fact that all four executables now take a much greater average time than previously to process
1 file does not reflect on the latest compilers' performance. It is due to the
fact that the later development branch of the test application has taken on lot of
parsing sophistication in the meantime and pays for it in speed. Only the ratios are
significant.
The points of note now are not arrestingly novel:
- GCC is indifferent to -O3 optimization
- clang benefits very marginally from -O3 optimization
- clang beats GCC by a similarly important margin at each level of optimization.
Comparing these results with those for GCC 4.7.2 and clang 3.2, it stands out that
GCC has clawed back about a quarter of clang's lead at each optimization level. But
since the test application has been heavily developed in the meantime one cannot
confidently attribute this to a catch-up in GCC's code-generation.
(This time, I have noted the application snapshot from which the timings were obtained
and can use it again.)
UPDATE: GCC 4.8.2 v clang 3.4
I finished the update for GCC 4.8.1 v Clang 3.3 saying that I would
stick to the same coan snaphot for further updates. But I decided
instead to test on that snapshot (rev. 301) and on the latest development
snapshot I have that passes its test suite (rev. 619). This gives the results a
bit of longitude, and I had another motive:
My original posting noted that I had devoted no effort to optimizing coan for
speed. This was still the case as of rev. 301. However, after I had built
the timing apparatus into the coan test harness, every time I ran the test suite
the performance impact of the latest changes stared me in the face. I saw that
it was often surprisingly big and that the trend was more steeply negative than
I felt to be merited by gains in functionality.
By rev. 308 the average processing time per input file in the test suite had
well more than doubled since the first posting here. At that point I made a
U-turn on my 10 year policy of not bothering about performance. In the intensive
spate of revisions up to 619 performance was always a consideration and a
large number of them went purely to rewriting key load-bearers on fundamentally
faster lines (though without using any non-standard compiler features to do so). It would be interesting to see each compiler's reaction to this
U-turn,
Here is the now familiar timings matrix for the latest two compilers' builds of rev.301:
coan - rev.301 results
| -O2 | -O3 |O2/O3|
----------|-----|-----|-----|
GCC-4.8.2 | 428 | 428 |1.00 |
----------|-----|-----|-----|
Clang-3.4 | 390 | 365 |1.07 |
----------|-----|-----|------
GCC/Clang | 1.1 | 1.17|
The story here is only marginally changed from GCC-4.8.1 and Clang-3.3. GCC's showing
is a trifle better. Clang's is a trifle worse. Noise could well account for this.
Clang still comes out ahead by -O2 and -O3 margins that wouldn't matter in most
applications but would matter to quite a few.
And here is the matrix for rev. 619.
coan - rev.619 results
| -O2 | -O3 |O2/O3|
----------|-----|-----|-----|
GCC-4.8.2 | 210 | 208 |1.01 |
----------|-----|-----|-----|
Clang-3.4 | 252 | 250 |1.01 |
----------|-----|-----|------
GCC/Clang |0.83 | 0.83|
Taking the 301 and the 619 figures side by side, several points speak out.
I was aiming to write faster code, and both compilers emphatically vindicate
my efforts. But:
GCC repays those efforts far more generously than Clang. At -O2
optimization Clang's 619 build is 46% faster than its 301 build: at -O3 Clang's
improvement is 31%. Good, but at each optimization level GCC's 619 build is
more than twice as fast as its 301.
GCC more than reverses Clang's former superiority. And at each optimization
level GCC now beats Clang by 17%.
Clang's ability in the 301 build to get more leverage than GCC from -O3 optimization
is gone in the 619 build. Neither compiler gains meaningfully from -O3.
I was sufficiently surprised by this reversal of fortunes that I suspected I
might have accidentally made a sluggish build of clang 3.4 itself (since I built
it from source). So I re-ran the 619 test with my distro's stock Clang 3.3. The
results were practically the same as for 3.4.
So as regards reaction to the U-turn: On the numbers here, Clang has done much
better than GCC at at wringing speed out of my C++ code when I was giving it no
help. When I put my mind to helping, GCC did a much better job than Clang.
I don't elevate that observation into a principle, but I take
the lesson that "Which compiler produces the better binaries?" is a question
that, even if you specify the test suite to which the answer shall be relative,
still is not a clear-cut matter of just timing the binaries.
Is your better binary the fastest binary, or is it the one that best
compensates for cheaply crafted code? Or best compensates for expensively
crafted code that prioritizes maintainability and reuse over speed? It depends on the
nature and relative weights of your motives for producing the binary, and of
the constraints under which you do so.
And in any case, if you deeply care about building "the best" binaries then you
had better keep checking how successive iterations of compilers deliver on your
idea of "the best" over successive iterations of your code.
Phoronix did some benchmarks about this, but it is about a snapshot version of Clang/LLVM from a few months back. The results being that things were more-or-less a push; neither GCC nor Clang is definitively better in all cases.
Since you'd use the latest Clang, it's maybe a little less relevant. Then again, GCC 4.6 is slated to have some major optimizations for Core 2 and i7, apparently.
I figure Clang's faster compilation speed will be nicer for original developers, and then when you push the code out into the world, Linux distro/BSD/etc. end-users will use GCC for the faster binaries.
There is very little overall difference between GCC 4.8 and clang 3.3 in terms of speed of the resulting binary. In most cases code generated by both compilers performs similarly. Neither of these two compilers dominates the other one.
Benchmarks telling that there is a significant performance gap between GCC and clang are coincidental.
Program performance is affected by the choice of the compiler. If a developer or a group of developers is exclusively using GCC then the program can be expected to run slightly faster with GCC than with clang, and vice versa.
From developer viewpoint, a notable difference between GCC 4.8+ and clang 3.3 is that GCC has the -Og command line option. This option enables optimizations that do not interfere with debugging, so for example it is always possible to get accurate stack traces. The absence of this option in clang makes clang harder to use as an optimizing compiler for some developers.
A peculiar difference I have noted on gcc 5.2.1 and clang 3.6.2 is
that if you have a critical loop like:
for (;;) {
if (!visited) {
....
}
node++;
if (!*node) break;
}
Then gcc will, when compiling with -O3 or -O2, speculatively
unroll the loop eight times. Clang will not unroll it at all. Through
trial and error I found that in my specific case with my program data,
the right amount of unrolling is five so gcc overshot and clang
undershot. However, overshotting was more detrimental to peformance so
gcc performed much worse here.
I have no idea if the unrolling difference is a general trend or
just something that was specific to my scenario.
A while back I wrote a few garbage
collectors to teach myself more
about performance optimization in C. And the results I got is in my
mind enough to slightly favor clang. Especially since garbage
collection is mostly about pointer chasing and copying memory.
The results are (numbers in seconds):
+---------------------+-----+-----+
|Type |GCC |Clang|
+---------------------+-----+-----+
|Copying GC |22.46|22.55|
|Copying GC, optimized|22.01|20.22|
|Mark & Sweep | 8.72| 8.38|
|Ref Counting/Cycles |15.14|14.49|
|Ref Counting/Plain | 9.94| 9.32|
+---------------------+-----+-----+
This is all pure C code and I make no claim about either compilers
performance when compiling C++ code.
On Ubuntu 15.10, x86.64, and an AMD Phenom(tm) II X6 1090T processor.
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