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<div class="moz-cite-prefix">On 10/16/2017 10:22 PM, Cohen, Elad2
via llvm-dev wrote:<br>
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<div class="WordSection1">
<p class="MsoNormal">Introduction<o:p></o:p></p>
<p class="MsoNormal">==========<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">We would like to add support for masked
vector signed/unsigned integer division and remainder in the
LLVM IR by introducing new target-independent intrinsics.<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">This follows similar work which was done
already for masked vector loads and stores -
<a
href="http://lists.llvm.org/pipermail/llvm-dev/2014-October/078059.html"
moz-do-not-send="true">http://lists.llvm.org/pipermail/llvm-dev/2014-October/078059.html</a><span
style="color:#1F497D">.</span><o:p></o:p></p>
<p class="MsoNormal">Another relevant reference is the masked
scatter/gather intrinsics discussion -
<a
href="http://lists.llvm.org/pipermail/llvm-dev/2014-December/079843.html"
moz-do-not-send="true">http://lists.llvm.org/pipermail/llvm-dev/2014-December/079843.html</a><span
style="color:#1F497D">.</span><o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">Motivation<o:p></o:p></p>
<p class="MsoNormal">=========<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">In the current state if the loop-vectorizer
decides that it should vectorize a loop which contains a
predicated integer division - it will vectorize the loop body
and scalarize the predicated division instruction into a
sequence of branches that guard scalar division operations. In
some cases the generated code for this will not be very
efficient. Speculating the divides using a non-masked vector
sdiv instruction is usually not an option due to the danger of
integer divide-by-zero.<o:p></o:p></p>
<p class="MsoNormal"> <o:p></o:p></p>
<p class="MsoNormal">With the addition of these hereby proposed
intrinsics the loop-vectorizer could concentrate on the vector
semantics rather than how to lower them, by generating the
masked intrinsics.<o:p></o:p></p>
<p class="MsoNormal">Initially the intrinsics will be scalarized
for all targets. This could be done by extending
scalarize-masked-mem-intrin to handle also division masked
intrinsics. Later the intrinsics could be optimized by:<o:p></o:p></p>
<p class="MsoListParagraph"
style="margin-left:43.5pt;text-indent:-18.0pt;mso-list:l0
level1 lfo1">
<!--[if !supportLists]--><span style="mso-list:Ignore">1.<span
style="font:7.0pt "Times New Roman"">
</span></span><!--[endif]--><span dir="LTR"></span>Lowering
of the intrinsics in the backend using different expansions
(for example converting to floating point and using masked
vector floating-point division instructions).<o:p></o:p></p>
<p class="MsoListParagraph"
style="margin-left:43.5pt;text-indent:-18.0pt;mso-list:l0
level1 lfo1">
<!--[if !supportLists]--><span style="mso-list:Ignore">2.<span
style="font:7.0pt "Times New Roman"">
</span></span><!--[endif]--><span dir="LTR"></span>Linking
the intrinsics to different vector math library
implementations.<o:p></o:p></p>
<p class="MsoListParagraph"
style="margin-left:43.5pt;text-indent:-18.0pt;mso-list:l0
level1 lfo1">
<!--[if !supportLists]--><span style="mso-list:Ignore">3.<span
style="font:7.0pt "Times New Roman"">
</span></span><!--[endif]--><span dir="LTR"></span>Scalarizing
the intrinsics at the backend possibly using target-specific
considerations.<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">Proposed Definition (The following example
is for masked signed division. The rest are similar)<o:p></o:p></p>
<p class="MsoNormal">========================================================================<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal"> ‘llvm.masked.sdiv’<s><o:p></o:p></s></p>
<p class="MsoNormal"> <o:p></o:p></p>
<p class="MsoNormal"> Syntax:<o:p></o:p></p>
<p class="MsoNormal"> <o:p></o:p></p>
<p class="MsoNormal"> An overloaded intrinsic. You can
use llvm.masked.sdiv on any vector with integer elements.<o:p></o:p></p>
<p class="MsoNormal"> <o:p></o:p></p>
<p class="MsoNormal"> declare <16 x i32>
@llvm.masked.sdiv.v16i32(<16 x i32> <a>, <16 x
i32> <b>, <16 x i1> <mask>, <16 x
i32> <passthru>)<o:p></o:p></p>
<p class="MsoNormal"> <o:p></o:p></p>
<p class="MsoNormal"> Overview:<o:p></o:p></p>
<p class="MsoNormal"> <o:p></o:p></p>
<p class="MsoNormal"> Returns the quotient of its two
operands per vector lane according to the provided mask. The
mask holds a bit for each vector lane, and is used to prevent
division in the masked-off lanes. The masked-off lanes in the
result vector are taken from the corresponding lanes of the
passthru operand.<o:p></o:p></p>
<p class="MsoNormal"> <o:p></o:p></p>
<p class="MsoNormal"> Arguments:<o:p></o:p></p>
<p class="MsoNormal"> <o:p></o:p></p>
<p class="MsoNormal"> The first two arguments must be
vectors of integer values. Both arguments must have identical
types. The third operand, mask, is a vector of boolean values
with the same number of elements as the first two. The fourth
is a pass-through value that is used to fill the masked-off
lanes of the result. The type of the passthru<s>
</s>operand is the same as the first two.<o:p></o:p></p>
<p class="MsoNormal"> <o:p></o:p></p>
<p class="MsoNormal"> Semantics:<o:p></o:p></p>
<p class="MsoNormal"> <o:p></o:p></p>
<p class="MsoNormal"> The ‘llvm.masked.sdiv’ intrinsic
is designed for conditional integer division of selected
vector elements in a single IR operation. The result of this
operation is equivalent to a regular vector 'sdiv' instruction
followed by a ‘select’ between the loaded and the passthru
values, predicated on the same mask. However, using this
intrinsic prevents divide-by-zero exceptions on division of
masked-off lanes. If any element in a turned-on lane of the
divisor is zero, the operation has undefined behavior. </p>
</div>
</blockquote>
<br>
You probably want to mention INT_MIN/-1 overflow here?<br>
<br>
----<br>
<br>
The alternative here is to refine the definition of "sdiv" in
LangRef; other arithmetic operations LLVM IR don't have undefined
behavior, and the primary reason "sdiv" has undefined behavior is
the unfortunate behavior of the x86 "IDIV" instruction. For
example, we could add a "nooverflow" bit to "sdiv", and say that
divide-by-zero has undefined behavior if the "nooverflow" bit is
present, and produces poison otherwise.<br>
<br>
-Eli<br>
<pre class="moz-signature" cols="72">--
Employee of Qualcomm Innovation Center, Inc.
Qualcomm Innovation Center, Inc. is a member of Code Aurora Forum, a Linux Foundation Collaborative Project</pre>
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