Hi,

On 18.10.2016 16:25, Jan Ziak wrote:
On Tue, Oct 18, 2016 at 3:12 PM, Nicolai Hähnle <nhaeh...@gmail.com> wrote:
On 18.10.2016 15:07, Jan Ziak wrote:
On Tue Oct 18 09:29:59 UTC 2016, Eero Tamminen wrote:
On 18.10.2016 01:07, Jan Ziak wrote:
- The total number of executed instructions goes down from 64.184 to
63.797
  giga-instructions when Mesa is compiled with "gcc -O0 ..."


Please don't do performance related decisions based on data from
compiling code with optimizations disabled.  Use -O2 or -O3 (or even
better, check both).

Options -O2 and -O3 interfere with profiling tools.

I will try using -Og the next time.

Just stop and use proper profiling tools like perf that can work with
optimized tools.

Valgrind/callgrind/cachegrind works also fine with optimized binaries.

All profiling tools lie, at least a bit. It's better to know their strengths and weaknesses so that one knows which ones complement each other. Perf is e.g. good at finding hotspots, Valgrind (callgrind) is more reliable in telling how they get called.

One may also needs GCC version from this decade. Really old GCC versions didn't inlude all debug info needed for debugging optimized binaries.


You may have to compile with -fno-omit-frame-pointer, but
anything more than that is going to distort your measurement results so much
that they're worthless.
>
The function from -O0, -O1, -O2 to number-of-instructions (or to
number-of-cycles) is in the vast majority of cases a monotonically
decreasing function.
>
> Are you saying that this rule does not hold in the case of this patch?

-O1 puts variables into registers instead of using them directly from stack like -O0 does. -O3 adds things like inlining and aliasing analysis. Only higher optimization levels do dead code analysis. Whether these increase or decrease code side, depends on the code.


        - Eero

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