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<p class=MsoNormal>Hi all,<o:p></o:p></p>
<p class=MsoNormal><o:p> </o:p></p>
<p class=MsoNormal>I’m getting more impressed by LLVM day by day, but
what’s a bit unclear to me now is the order of optimization passes, and their
performance. I think I have a pretty solid understanding of what each pass does
at a high level, but I couldn’t find any documentation about how they
interact at a lower level.<o:p></o:p></p>
<p class=MsoNormal><o:p> </o:p></p>
<p class=MsoNormal>I’d like to use LLVM for generating high-performance
stream processing code at run-time. Obviously the resulting code should be as
optimized as possible, but performing the optimizations themselves should also
be very fast. The code I’m compiling is comparable to C (without any
exception handling or garbage collection, so none of the related passes are
needed). My first attempt at collecting useful optimizations looks like this:<o:p></o:p></p>
<p class=MsoNormal><o:p> </o:p></p>
<p class=MsoNormal style='text-autospace:none'><span style='font-size:10.0pt;
font-family:"Courier New"'>passManager->add(<span style='color:blue'>new</span>
TargetData(*executionEngine->getTargetData()));<o:p></o:p></span></p>
<p class=MsoNormal style='text-autospace:none'><span style='font-size:10.0pt;
font-family:"Courier New"'>passManager->add(createScalarReplAggregatesPass());
// Convert to SSA form<o:p></o:p></span></p>
<p class=MsoNormal style='text-autospace:none'><span style='font-size:10.0pt;
font-family:"Courier New"'>passManager->add(createSCCPPass());
// Propagate constants<o:p></o:p></span></p>
<p class=MsoNormal style='text-autospace:none'><span style='font-size:10.0pt;
font-family:"Courier New"'>passManager->add(createInstructionCombiningPass());
// Peephole optimization<o:p></o:p></span></p>
<p class=MsoNormal style='text-autospace:none'><span style='font-size:10.0pt;
font-family:"Courier New"'>passManager->add(createDeadStoreEliminationPass());
// Dead store elimination<o:p></o:p></span></p>
<p class=MsoNormal><span style='font-size:10.0pt;font-family:"Courier New"'>passManager->add(createAggressiveDCEPass());
// Aggressive dead code elimination<o:p></o:p></span></p>
<p class=MsoNormal style='text-autospace:none'><span style='font-size:10.0pt;
font-family:"Courier New"'>passManager->add(createCFGSimplificationPass());
// Control-flow optimization<o:p></o:p></span></p>
<p class=MsoNormal><o:p> </o:p></p>
<p class=MsoNormal>I have several questions about this:<o:p></o:p></p>
<p class=MsoNormal><o:p> </o:p></p>
<p class=MsoNormal>1) Does ScalarReplAggregates totally superscede
PromoteMemoryToRegister? I think I need it to optimize small arrays, but what
is the expected added complexity?<o:p></o:p></p>
<p class=MsoNormal>2) Does SCCP also eliminate multiplying/dividing by 1 and
adding/subtracting 0?<o:p></o:p></p>
<p class=MsoNormal>3) Is it arbitrary where to place InstructionCombining? Is
there a better order?<o:p></o:p></p>
<p class=MsoNormal>4) Is DeadStoreElimination still necessary when we have
AggressiveDCE?<o:p></o:p></p>
<p class=MsoNormal>5) What are the tradeoffs between the different dead code
elimination variants (why not always use the aggressive one)? <o:p></o:p></p>
<p class=MsoNormal>6) Is there a better place for CFGSimplification? Should I
perform it at multiple points?<o:p></o:p></p>
<p class=MsoNormal><o:p> </o:p></p>
<p class=MsoNormal>Also, my code will frequently have vectors, that are either
initialized to all 0.0 or 1.0. This offers a lot of opportunity for eliminating
many multiplications and additions, but I was wondering which passes I need for
this (probably a Reassociate pass, what else)? And I also wonder whether these
passes actually work with vectors?<o:p></o:p></p>
<p class=MsoNormal><o:p> </o:p></p>
<p class=MsoNormal>Is there any other highly recommended pass for this kind of
applications that I’m forgetting? Any passes that I better avoid due to poor
gains and long optimization time?<o:p></o:p></p>
<p class=MsoNormal><o:p> </o:p></p>
<p class=MsoNormal>Sorry for the many question marks. :-) I don’t expect
there is an absolute definite answer to each of them, but some guidelines and
insights would be very welcome and much appreciated.<o:p></o:p></p>
<p class=MsoNormal><o:p> </o:p></p>
<p class=MsoNormal>Thanks!<o:p></o:p></p>
<p class=MsoNormal><o:p> </o:p></p>
<p class=MsoNormal>Nicolas Capens<o:p></o:p></p>
<p class=MsoNormal><o:p> </o:p></p>
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