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<p class="MsoNormal"><span style="font-size:12.0pt">Doug<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:12.0pt"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:12.0pt">Good stuff. <o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:12.0pt"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:12.0pt">Do you know about ticky-ticky profiling? It is /guaranteed/ not to affect optimisation. Moreover you can mix modules compiled with and without profiling. But it’s very low level.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:12.0pt"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:12.0pt">I had not thought of doing /cost-centre/ profiling after optimisation. That’s an interesting thought. You’d still have to compile the entire program with profiling (because runtime representations change).
But you’d get the call-stack information etc that cost centres give you, but with a guarantee that optimisation was unaffected. It’s a sort of middle ground.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:12.0pt"><o:p> </o:p></span></p>
<p class="MsoNormal" style="margin-left:36.0pt">Is the end of corePrepPgm the right place to insert the cost-centres? I chose it<br>
because it can't affect any core optimizations if it's last, but perhaps it<br>
could be earlier, or perhaps it needs to act on Stg?<br>
<br>
<span style="font-size:12.0pt"><o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:12.0pt">I’d be inclined to do it on STG, just before codegen. Because it really is a codgen tactic “compile this with cost centres please”.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:12.0pt"><o:p> </o:p></span></p>
<p class="MsoNormal" style="margin-left:36.0pt">I need to strip (probably only non-user) ticks out of unfoldings before they are<br>
substituted into a module that uses -fprof-core.<span style="font-size:12.0pt"><o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:12.0pt"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:12.0pt">Why? If you are only adding these cost centres after CorePrep, or in STG, they will never show up in unfoldings anyway.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:12.0pt"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:12.0pt">Simon<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:12.0pt"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:12.0pt"><o:p> </o:p></span></p>
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<p class="MsoNormal"><b><span lang="EN-US">From:</span></b><span lang="EN-US"> ghc-devs [mailto:ghc-devs-bounces@haskell.org]
<b>On Behalf Of </b>Douglas Wilson<br>
<b>Sent:</b> 14 November 2017 22:34<br>
<b>To:</b> ghc-devs@haskell.org<br>
<b>Subject:</b> New core profiling mode<o:p></o:p></span></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:6.0pt;margin-right:0cm;margin-bottom:6.0pt;margin-left:0cm">
Hi ghc-devs,<br>
<br>
I've been working on a new mode of adding cost-centres to programs and I'd<br>
like to ask some questions and solicit some feedback. The code is here [fn:1],<br>
it works, provided one enables -fprof-core on all modules.<br>
<br>
I've recently been trying to pick some low hanging fruit from ghc compilation<br>
performance. A common frustration was in the difference between profiled and<br>
non-profiled builds. Often I thought had I found a problem in the profiled<br>
build, only to find it was optimized away in the non-profiled build. Several<br>
times an issue was tail-calls not happening in profiled builds.<br>
<br>
To solve this problem I've been working on a new way of inserting cost-centres:<br>
adding them to core after simplification (currently at the end of corePrepPgm)<br>
rather than adding them to HsSyn before simplification. This makes it harder to<br>
map cost-centres into source code (You have to -ddump-prep currently), but in<br>
exchange you are profiling the same core program as the non-profilied build.<br>
<br>
I intend to investigate whether I can use SourceNotes to create SrcSpans for the<br>
generated cost-centres to somewhat alleviate the need to inspect dumped core.<br>
<br>
There are several new flags:<br>
<br>
-fprof-core: Enables the aforementioned mode. This is mutually exclusive with<br>
-fprof-auto etc.<br>
<br>
-fprof-core-drop-ticks: Non-user ticks are dropped from unfoldings(though I<br>
don't know how to do this yet).<br>
<br>
-fprof-core-tick-binds: ticks are inserted around the RHS of bindings (except<br>
top-level unlifted bindings).<br>
<br>
-fprof-core-tick-cases: ticks are inserted around the scrutinees of cases.<br>
<br>
-fprof-core-tick-alts: ticks are inserted around Alt expressions (unless there<br>
is only one).<br>
<br>
Some questions:<br>
<br>
I need to strip (probably only non-user) ticks out of unfoldings before they are<br>
substituted into a module that uses -fprof-core. Where is the right place to do<br>
this? I need inlining to proceed exactly as if the ticks were not present,<br>
however I don't want to strip ticks when the unfoldings are created as other<br>
modules may still need them.<br>
<br>
Is the end of corePrepPgm the right place to insert the cost-centres? I chose it<br>
because it can't affect any core optimizations if it's last, but perhaps it<br>
could be earlier, or perhaps it needs to act on Stg?<br>
<br>
Do you have any examples of programs for which existing profiling tools are<br>
inadequate due to how cost-centres affect simplification? There is an example in<br>
#12893 but something self-contained would be great!<br>
<br>
Regards,<br>
Doug Wilson<o:p></o:p></p>
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