llvm dev - Jan 2022 - [RDF] Question about function argument registers on X86

Hi,

Given the function:

long simpleExample(long x) {
  return x;
}

RDF produces the following data-flow graph:

DFG dump:[
f1: Function: simpleExample
b2: --- %bb.0 --- preds(0):   succs(0):
p8: phi [+d9<DIH:0000000000000001>(,,u15"):]
p10: phi [+d11<DIL:0000000000000001>(,,u14"):]
p12: phi [+d13<HDI:0000000000000001>(,,u5"):]
s3: COPY [d4<RAX>(,,u7):, u5"<RDI>(+d13):,
u14"<RDI>(+d11):, u15"<RDI>(+d9):]
s6: RET [u7<RAX>!(d4):]

]

Why is the argument in RDI split into three lanes that do not fully span RDI? It
would seem more natural to just have a single phi for RDI.

I think the RDF code responsible for this behavior is located here:
https://github.com/llvm/llvm-project/blob/55d887b833646baeea0e3371fd2cbbd7550a8d4d/llvm/lib/CodeGen/RDFGraph.cpp#L903,
but I admit I do not fully understand what is going on.

Thanks in advance,

Scott Constable

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Hi Scott,

It used to be the case early on that we'd create an entry for the register
as it was specified, but I've been running into all sorts of complications
when calculating liveness.  Specifically, when defs/uses of "large"
registers were intermingled with defs/uses of their sub-registers, various
"interesting" scenarios showed up.  They had to do with representing
unions/intersections of registers in terms of actual registers[1]---that was the
reason for switching to tracking register units instead (see RegisterAggr
class).
The consequence of that is that the graph looks more cluttered, but things are
easier to express accurately.

[1] This was before lane masks became popular.  Say we have
AX = def
AH = clobber
Here AX is still "defined" since some part of it is defined.  Now, if
we know that AX is "defined" then what can we say about it after
executing "AL = clobber"?  Turns out that this is difficult to answer,
because we'd have to know the history of how AX was modified, which
isn't always analyzed in full: (RDF stop def traversal at a PHI node).


--
Krzysztof Parzyszek  kparzysz at quicinc.com<mailto:kparzysz at
quicinc.com>   AI tools development

From: llvm-dev <llvm-dev-bounces at lists.llvm.org> On Behalf Of
Constable, Scott D via llvm-dev
Sent: Saturday, January 22, 2022 3:24 PM
To: llvm-dev at lists.llvm.org
Subject: [llvm-dev] [RDF] Question about function argument registers on X86


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Hi,

Given the function:

long simpleExample(long x) {
  return x;
}

RDF produces the following data-flow graph:

DFG dump:[
f1: Function: simpleExample
b2: --- %bb.0 --- preds(0):   succs(0):
p8: phi [+d9<DIH:0000000000000001>(,,u15"):]
p10: phi [+d11<DIL:0000000000000001>(,,u14"):]
p12: phi [+d13<HDI:0000000000000001>(,,u5"):]
s3: COPY [d4<RAX>(,,u7):, u5"<RDI>(+d13):,
u14"<RDI>(+d11):, u15"<RDI>(+d9):]
s6: RET [u7<RAX>!(d4):]

]

Why is the argument in RDI split into three lanes that do not fully span RDI? It
would seem more natural to just have a single phi for RDI.

I think the RDF code responsible for this behavior is located here:
https://github.com/llvm/llvm-project/blob/55d887b833646baeea0e3371fd2cbbd7550a8d4d/llvm/lib/CodeGen/RDFGraph.cpp#L903,
but I admit I do not fully understand what is going on.

Thanks in advance,

Scott Constable

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