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<p class="MsoNormal">Yes, FPMathOperator isn’t part of the class hierarchy. I would interpret “increasing the size of FPMathOperator” as shorthand for increasing the size of ContstantExpr and the relevant subclasses of Instruction. (Exactly which subclasses
of Instruction gets a little sticky because most floating-point operations are represented by BinaryOperator, which is also used for integer arithmetic, but it’s probably solvable somehow.)<o:p></o:p></p>
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<p class="MsoNormal">-Eli<o:p></o:p></p>
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<p class="MsoNormal"><b>From:</b> llvm-dev <llvm-dev-bounces@lists.llvm.org> <b>On Behalf Of
</b>Sanjay Patel via llvm-dev<br>
<b>Sent:</b> Monday, March 18, 2019 5:16 PM<br>
<b>To:</b> Sanjoy Das <sanjoy@playingwithpointers.com><br>
<b>Cc:</b> llvm-dev <llvm-dev@lists.llvm.org><br>
<b>Subject:</b> [EXT] Re: [llvm-dev] [RFC] Making space for a flush-to-zero flag in FastMathFlags<o:p></o:p></p>
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<p class="MsoNormal">I don't have any objections to increasing the size of FPMathOperator, but I also don't know what perf impact that would have.<o:p></o:p></p>
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<p class="MsoNormal">I made this comment in D39304:<o:p></o:p></p>
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<p class="MsoNormal">"I don't think we can just add a field to FPMathOperator because Operator is not intended to be instantiated."<o:p></o:p></p>
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<p class="MsoNormal">That could just be me not understanding the class hierarchy?<o:p></o:p></p>
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<p class="MsoNormal">On Mon, Mar 18, 2019 at 11:56 AM Sanjoy Das <<a href="mailto:sanjoy@playingwithpointers.com">sanjoy@playingwithpointers.com</a>> wrote:<o:p></o:p></p>
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<p class="MsoNormal">On Mon, Mar 18, 2019 at 9:31 AM Sanjay Patel <<a href="mailto:spatel@rotateright.com" target="_blank">spatel@rotateright.com</a>> wrote:<br>
><br>
> We knew the day when we needed another FMF bit was coming back in:<br>
> <a href="https://reviews.llvm.org/D39304" target="_blank">https://reviews.llvm.org/D39304</a><br>
> ...it was just a question of 'when'. :)<br>
><br>
> I'm guessing that an FTZ bit won't be the last new bit needed if we consider permutations between strict FP and fast-math. Even without that, denormals-as-zero (DAZ) might also be useful?<br>
> So rather than continuing to carve these out bit-by-bit, it's worth considering a more general solution: instruction-level metadata.<br>
><br>
> IIUC, the main argument for making FMF part of the instruction was that per-instruction metadata gets expensive if we're applying it to a significant chunk of the instructions.<br>
> But let's think about that - even the most FP-heavy code tops out around 10% FP math ops out of the total instruction count. Typical FP benchmark code is only 2-5% FP ops. The rest is the same load/store/control-flow/ALU stuff found in integer code.<br>
<br>
If this is true, what do you think about option (1)? It might be<br>
simpler to increase the size of FPMathOperator by a word, giving us 64<br>
more bits of fastmath flags. We could also have this extra word in<br>
only those instances of FPMathOperator that have a non-zero<br>
FastMathFlags (this would force us to remove setFastMathFlags since<br>
we'd need to know the contents of FastMathFlags at Instruction<br>
construction time).<br>
<br>
-- Sanjoy<o:p></o:p></p>
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