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Hi Simon,
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<div class="">Thanks for your feedback! See my comments inline.
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<div class="">On 5 Apr 2019, at 09:47, Simon Pilgrim via llvm-dev <<a href="mailto:llvm-dev@lists.llvm.org" class="">llvm-dev@lists.llvm.org</a>> wrote:</div>
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On 05/04/2019 09:37, Simon Pilgrim via llvm-dev wrote:<br class="">
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<blockquote type="cite" cite="mid:d306cf98-1225-732d-8016-7e882b5136b1@redking.me.uk" style="font-family: Helvetica; font-size: 12px; font-style: normal; font-variant-caps: normal; font-weight: normal; letter-spacing: normal; orphans: auto; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; widows: auto; word-spacing: 0px; -webkit-text-size-adjust: auto; -webkit-text-stroke-width: 0px; background-color: rgb(255, 255, 255); text-decoration: none;" class="">
<div class="moz-cite-prefix">On 04/04/2019 14:11, Sander De Smalen wrote:<br class="">
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<blockquote type="cite" cite="mid:67D8F282-9E37-473F-9973-AA981D992711@arm.com" class="">
<div class="WordSection1" style="page: WordSection1;"><span style="font-size: 11pt;" class="">Proposed change:<o:p class=""></o:p></span>
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<span style="font-size: 11pt;" class="">----------------------------<o:p class=""></o:p></span></div>
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<span style="font-size: 11pt;" class="">In this RFC I propose changing the intrinsics for llvm.experimental.vector.reduce.fadd and llvm.experimental.vector.reduce.fmul (see options A and B). I also propose renaming the 'accumulator' operand to 'start value'
because for fmul this is the start value of the reduction, rather than a value to which the fmul reduction is accumulated into.<o:p class=""></o:p></span></div>
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<span style="font-size: 11pt;" class=""><o:p class=""> </o:p></span></div>
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<span style="font-size: 11pt;" class="">[Option A] Always using the start value operand in the reduction (<a href="https://reviews.llvm.org/D60261" moz-do-not-send="true" style="color: rgb(149, 79, 114); text-decoration: underline;" class="">https://reviews.llvm.org/D60261</a>)<o:p class=""></o:p></span></div>
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<span style="font-size: 11pt;" class=""><o:p class=""> </o:p></span></div>
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<span style="font-size: 11pt;" class=""> declare float @llvm.experimental.vector.reduce.v2.fadd.f32.v4f32(float %start_value, <4 x float> %vec)<o:p class=""></o:p></span></div>
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<span style="font-size: 11pt;" class=""><o:p class=""> </o:p></span></div>
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<span style="font-size: 11pt;" class="">This means that if the start value is 'undef', the result will be undef and all code creating such a reduction will need to ensure it has a sensible start value (e.g. 0.0 for fadd, 1.0 for fmul). When using 'fast' or
‘reassoc’ on the call it will be implemented using an unordered reduction, otherwise it will be implemented with an ordered reduction. Note that a new intrinsic is required to capture the new semantics. In this proposal the intrinsic is prefixed with a 'v2'
for the time being, with the expectation this will be dropped when we remove 'experimental' from the reduction intrinsics in the future.</span><span style="font-size: 11pt; font-family: "MS Gothic";" class=""><o:p class=""></o:p></span></div>
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<span style="font-size: 11pt;" class=""><o:p class=""> </o:p></span></div>
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<span style="font-size: 11pt;" class="">[Option B] Having separate ordered and unordered intrinsics (<a href="https://reviews.llvm.org/D60262" moz-do-not-send="true" style="color: rgb(149, 79, 114); text-decoration: underline;" class="">https://reviews.llvm.org/D60262</a>).<o:p class=""></o:p></span></div>
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<span style="font-size: 11pt;" class=""><o:p class=""> </o:p></span></div>
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<span style="font-size: 11pt;" class=""> declare float @llvm.experimental.vector.reduce.ordered.fadd.f32.v4f32(float %start_value, <4 x float> %vec)<o:p class=""></o:p></span></div>
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<span style="font-size: 11pt;" class=""> declare float @llvm.experimental.vector.reduce.unordered.fadd.f32.v4f32(<4 x float> %vec)<o:p class=""></o:p></span></div>
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<span style="font-size: 11pt;" class=""><o:p class=""> </o:p></span></div>
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<span style="font-size: 11pt;" class="">This will mean that the behaviour is explicit from the intrinsic and the use of 'fast' or ‘reassoc’ on the call has no effect on how that intrinsic is lowered. The ordered reduction intrinsic will take a scalar start-value
operand, where the unordered reduction intrinsic will only take a vector operand.<o:p class=""></o:p></span></div>
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<span style="font-size: 11pt;" class=""><o:p class=""> </o:p></span></div>
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<span style="font-size: 11pt;" class="">Both options auto-upgrade the IR to use the new (version of the) intrinsics. I'm personally slightly in favour of [Option B], because it better aligns with the definition of the SelectionDAG nodes and is more explicit
in its semantics. We also avoid having to use an artificial 'v2' like prefix to denote the new behaviour of the intrinsic.<o:p class=""></o:p></span></div>
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<p class="">Do we have any targets with instructions that can actually use the start value? TBH I'd be tempted to suggest we just make the initial extractelement/fadd/insertelement pattern a manual extra stage and avoid having having that argument entirely.<span class="Apple-converted-space"> </span><br class="">
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ARM SVE has the FADDA instruction for strict fadd reductions (see for example test/MC/AArch64/SVE/fadda.s). This instruction takes an explicit start-value operand. The reduction intrinsics were originally introduced for SVE where we modelled the fadd/fmul reductions
with this instruction in mind.</div>
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<div style="margin: 0px; font-stretch: normal; line-height: normal;" class=""><font face="Helvetica Neue" class="">Just to clarify, is this what you are suggesting regarding extract/fadd/insert?<br class="">
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%first = extractelement <4 x float> %input, i32 0<br class="">
%first.new = fadd float %start, %first<br class="">
%input.new = insertelement <4 x float> %input, float %first.new, i32 0<br class="">
%red = call float @llvm.experimental.vector.reduce.ordered.fadd.f32.v4f32(<4 x float> %input.new)<br class="">
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My only reservation here is that LLVM might obfuscate this code so that CodeGen couldn't easily match the extract/fadd/insert pattern, thus adding the extra fadd instruction. This could for example happen if the loop would be rotated/pipelined to load the next
iteration and doing the first 'fadd' before the next iteration. </font><span style="font-family: "Helvetica Neue";" class="">In such case having the extra operand would be more descriptive.</span></div>
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<blockquote type="cite" cite="mid:67D8F282-9E37-473F-9973-AA981D992711@arm.com" class="">
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<span style="font-size: 11pt;" class="">Further efforts:<o:p class=""></o:p></span></div>
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<span style="font-size: 11pt;" class="">----------------------------<o:p class=""></o:p></span></div>
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<span style="font-size: 11pt;" class="">Here a non-exhaustive list of items I think work towards making the intrinsics non-experimental:</span><span lang="EN-US" style="font-size: 11pt; font-family: "MS Gothic";" class="">
</span><span style="font-size: 11pt;" class=""><o:p class=""></o:p></span></div>
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<span style="font-size: 11pt;" class="">Adding SelectionDAG legalization for the _STRICT reduction SDNodes. After some great work from Nikita in D58015, unordered reductions are now legalized/expanded in SelectionDAG, so if we add expansion in SelectionDAG
for strict reductions this would make the ExpandReductionsPass redundant.<o:p class=""></o:p></span></li><li class="MsoListParagraph" style="margin: 0cm 0cm 0.0001pt -18pt; font-size: 12pt; font-family: Calibri, sans-serif;">
<span style="font-size: 11pt;" class="">Better enforcing the constraints of the intrinsics (see<span class="Apple-converted-space"> </span><a href="https://reviews.llvm.org/D60260" moz-do-not-send="true" style="color: rgb(149, 79, 114); text-decoration: underline;" class="">https://reviews.llvm.org/D60260</a><span class="Apple-converted-space"> </span>).</span><span lang="EN-US" style="font-size: 11pt;" class="">
</span><span style="font-size: 11pt;" class=""><o:p class=""></o:p></span></li><li class="MsoListParagraph" style="margin: 0cm 0cm 0.0001pt -18pt; font-size: 12pt; font-family: Calibri, sans-serif;">
<span style="font-size: 11pt;" class="">I think we'll also want to be able to overload the result operand based on the vector element type for the intrinsics having the constraint that the result type must match the vector element type. e.g. dropping the redundant
'i32' in:</span><span style="font-size: 11pt;" class=""><br class="">
<span class="Apple-converted-space"> </span></span><span style="font-size: 11pt;" class="">i32 @llvm.experimental.vector.reduce.and.i32.v4i32(<4 x i32> %a) => i32 @llvm.experimental.vector.reduce.and.v4i32(<4 x i32> %a)<o:p class=""></o:p></span></li></ul>
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<span style="font-size: 11pt;" class="">since i32 is implied by <4 x i32>. This would have the added benefit that LLVM would automatically check for the operands to match.</span><span lang="EN-US" style="font-size: 11pt; font-family: "MS Gothic";" class="">
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<p class="">Won't this cause issues with overflow? Isn't the point of an add (or mul....) reduction of say, <64 x i8> giving a larger (i32 or i64) result so we don't lose anything? I agree for bitop reductions it doesn't make sense though.<br class="">
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<span style="caret-color: rgb(0, 0, 0); font-family: Helvetica; font-size: 12px; font-style: normal; font-variant-caps: normal; font-weight: normal; letter-spacing: normal; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; word-spacing: 0px; -webkit-text-stroke-width: 0px; background-color: rgb(255, 255, 255); text-decoration: none; float: none; display: inline !important;" class="">Sorry
- I forgot to add: which asks the question - should we be considering signed/unsigned add/mul and possibly saturation reductions?</span><br style="caret-color: rgb(0, 0, 0); font-family: Helvetica; font-size: 12px; font-style: normal; font-variant-caps: normal; font-weight: normal; letter-spacing: normal; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; word-spacing: 0px; -webkit-text-stroke-width: 0px; background-color: rgb(255, 255, 255); text-decoration: none;" class="">
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The current intrinsics explicitly specify that:</div>
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"The return type matches the element-type of the vector input"</div>
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This was done to avoid having explicit signed/unsigned add reductions, reasoning that zero- and sign-extension can be done on the input values to the reduction. We had a bit of debate on this internally, and it would come down to similar reasons as for the
extra 'start value' operand to fadd reductions. I think we'd welcome the signed/unsigned variants as they would be more descriptive and would safeguard the code from transformations that make it difficult to fold the sign/zero extend into the operation during
CodeGen. The downside however is that for signed/unsigned add reductions it would mean that both operations are the same when the result type equals the element type.</div>
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<div style="margin: 0px; font-stretch: normal; line-height: normal;" class="">Saturating vector reductions sound sensible, but are there any targets that implement these at the moment?</div>
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