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<p class="MsoNormal">Hi everyone,<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">Melanie Blower recently submitted a change that was intended to make the default set of floating point options in clang be consistent with the options that would be set by the -ffp-model=precise umbrella option. The only change needed was
to make the default for fp-contract “on” instead of “off”. While not a trivial change, we thought this was reasonable, since fp-contract=on only allows contraction that is allowed by the language standard. Unfortunately, this change unleashed a surprising
number of problems.<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">The most surprising problem, to me at least, was that this change caused FMA instructions to be generated at -O0.<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">There are a couple of things that need to be sorted out here, but I’d like to start with the -O0 behavior. Consider the following scenario, which was possible even before the recent change:<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">--------<o:p></o:p></p>
<p class="MsoNormal">test.c<o:p></o:p></p>
<p class="MsoNormal">--------<o:p></o:p></p>
<p class="MsoNormal">double f(double a, double b, double c) {<o:p></o:p></p>
<p class="MsoNormal"> return a * b + c;<o:p></o:p></p>
<p class="MsoNormal">}<o:p></o:p></p>
<p class="MsoNormal">--------<o:p></o:p></p>
<p class="MsoNormal">clang -c -O0 -ffp-contract=on test.c<o:p></o:p></p>
<p class="MsoNormal">--------<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">Since clang 5.0 this has produced a call to llvm.fmuladd, which for targets that support FMA will generally result in an FMA instruction. Arguably this is what the user asked for, since they explicitly enabled fp-contract. On the other
hand, it is also an optimization, which they said they did not want. As a point of comparison, specifying -ffast-math will cause the front end to attach the “fast” flag to math operations (which also allows contraction), but will not lead to FMA formation.<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">What should we do with this? I see two possible solutions:<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">1. The driver should not pass the -ffp-contract=on flag by default at -O0 (still allows fmuladd formation if the user specifies -ffp-contract=on)<o:p></o:p></p>
<p class="MsoNormal">2. The front end should not form the llvm.fmuladd intrinsic at -O0<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">The second option seems preferable to me, but I don’t know how unnatural it might be for the front end to respond to optimization level.<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">Apart from the -O0 dilemma, the change in default fp-contract behavior seems to have led to other problems. It introduced some performance regressions in LNT on x86 and some accuracy-related test failures on PowerPC. There are likely other
issues that I just haven’t heard about. So, I guess we should talk about whether we really want to enable this by default when optimizations are enabled. I don’t know anything about the PowerPC issues. I looked at the top x86 performance regression and it
seems that the introduction of the fmuladd intrinsic changed the decision of the loop unroller (the key loop is unrolled by 8 instead of 4). I’m inclined to regard that as a fluke of the test case or possibly a problem in the loop unroller, but I wouldn’t
see it as a reason to prefer to disable FP contraction.<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">FWIW, the fp-model option was intended to provide the same functionality as the /fp option in the MSVC compiler. The MSVC /fp:precise option enables FP contraction.<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">Input here would be 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">Andy<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
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