<div dir="ltr">+1 from Cray. <div><br></div><div>If we are talking about "having a single option to control most basic FP options" in general, I'd also like to see a finer granularity for -fp-model=fast, like Intel's own fast=[1|2]. Perhaps they just become their own options? I don't have a full list of proposed optimizations (and/or other existing flags) that would be tied for each of the finer grained levels, but at a high level, for example:</div><div><div><br> - precise (as described)<div> - strict (as described)</div><div> - optimize (something between precise and fast)</div><div> - fast (as described)</div><div> - aggressive (even more aggressive FP optimization than fast)</div><div> - except/except- (as described)</div></div><div><br></div><div>This would maintain reasonable command line compatibility but also allow for easier tuning (and debugging) of codes that are numerically sensitive at a global user level.</div></div></div><br><div class="gmail_quote"><div dir="ltr" class="gmail_attr">On Wed, May 29, 2019 at 10:37 AM Blower, Melanie via llvm-dev <<a href="mailto:llvm-dev@lists.llvm.org">llvm-dev@lists.llvm.org</a>> wrote:<br></div><blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex"><br>
Intel would like to contribute a patch to implement support for these Intel- and Microsoft -fp options. This message is to describe the options and request feedback from the community.<br>
-fp-model=[precise|strict|fast|except[-]] and<br>
-fp-speculation=[fast|strict|safe]<br>
<br>
This contribution would dovetail with the llvm patch "Teach the IRBuilder about constrained fadd and friends" which is under review here, <a href="https://reviews.llvm.org/D53157/new/" rel="noreferrer" target="_blank">https://reviews.llvm.org/D53157/new/</a>. I have a patch ready to review that works with D53157. The motivation for providing these is that having a single option to control most basic FP options is better and easier to understand for users.<br>
<br>
The option settings -fp-model=[precise|strict|fast|except] are supported by both ICC and CL. The fp-speculation option is supported only by ICC. The CL and ICC -fp-model option is documented on these pages: <br>
<a href="https://docs.microsoft.com/en-us/cpp/build/reference/fp-specify-floating-point-behavior?view=vs-2019" rel="noreferrer" target="_blank">https://docs.microsoft.com/en-us/cpp/build/reference/fp-specify-floating-point-behavior?view=vs-2019</a><br>
<a href="https://software.intel.com/en-us/cpp-compiler-developer-guide-and-reference-fp-model-fp" rel="noreferrer" target="_blank">https://software.intel.com/en-us/cpp-compiler-developer-guide-and-reference-fp-model-fp</a><br>
<br>
Currently, clang's default behavior corresponds to -fp-model=precise. Clang/llvm support for -fp-model=[strict|except] is being developed in the D53157 patch, and there is current llvm support for the fast settings by using the fast math flags llvm::FastMathFlags. Note: the clang-cl wrapper to support Microsoft options has simplified support for these options by mapping /fp-model=except to ftrapping-math, fp-mdel=fast to ffast-math, fp-model=precise and fp-model=strict to fno-fast-math (see clang/Driver/CLCompatOptions.td).<br>
<br>
According to the online options documentation, you can combine except[-] with [precise|strict|fast], but combining both fast and except is not a valid setting (the Driver will emit an error). <br>
precise - Disables optimizations that are not value-safe on floating-point data, although FP contraction is enabled.<br>
strict - Enables precise and except, disables contractions (FMA), and enables pragma stdc fenv_access. <br>
fast - Equivalent to -ffast-math<br>
except/except- - Determines whether strict floating-point exception semantics are honored.<br>
<br>
The ICC option -fp-speculation is described here,<br>
<a href="https://software.intel.com/en-us/cpp-compiler-developer-guide-and-reference-fp-speculation-qfp-speculation" rel="noreferrer" target="_blank">https://software.intel.com/en-us/cpp-compiler-developer-guide-and-reference-fp-speculation-qfp-speculation</a><br>
These are the meanings of the fp-speculation settings:<br>
fast - Tells the compiler to speculate on floating-point operations. This is equivalent to "fpexcept.ignore" in the constrained intrinsics review D53157.<br>
strict - Tells the compiler to disable speculation on floating-point operations. This is equivalent to "fpexcept.strict" in the constrained intrinsics review D53157.<br>
safe - Tells the compiler to disable speculation if there is a possibility that the speculation may cause a floating-point exception. This is equivalent to "fpexcept.maytrap" in the constrained intrinsics review D53157.<br>
<br>
<br>
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</blockquote></div>