[LLVMdev] Naryreassociate vs reassociate

[Jingyue Wu]

Hi Daniel,

I presume you mean, instead of assigning function arguments distinct ranks (
http://llvm.org/docs/doxygen/html/Reassociate_8cpp_source.html#l00282), we
should group function arguments in favor of existing pairings. You are not
suggesting discarding the entire ranking system, right?

I'll look into how that works on my benchmarks. AFAIK, we encountered some
cases that seem beyond the fix you suggested. These cases involve
constants, and I attached one reduced example in
https://llvm.org/bugs/show_bug.cgi?id=22357.

void foo(int a, int b, int c, int *x, int *y) {
  *x = (a + b);
  *y = (a + 2) + b;
}

Reassociate assigns constants a lower rank than variables, which prevents
Reassociate from transforming the above example to

*x = a + b;
*y = (a + b) + 2;

Jingyue

On Mon, May 4, 2015 at 4:45 PM Daniel Berlin <dberlin at dberlin.org> wrote:

> Whoops, forgot llvmdev
>
>
> On Mon, May 4, 2015 at 4:12 PM, Daniel Berlin <dberlin at dberlin.org> wrote:
> > So i started by looking at naryreassociate, whose pass
> > description/reason listed for doing it is actually describes bug in
> > reassociate, and discovered that, in fact, reassociate seems broken,
> > and should be doing the right thing on most of your testcases.
> >
> > Let's take nary-add.ll, left_reassociate
> >
> > ; RUN: opt < %s -nary-reassociate -S | FileCheck %s
> >
> > target datalayout = "e-i64:64-v16:16-v32:32-n16:32:64"
> >
> > declare void @foo(i32)
> >
> > ; foo(a + c);
> > ; foo((a + (b + c));
> > ;   =>
> > ; t = a + c;
> > ; foo(t);
> > ; foo(t + b);
> > define void @left_reassociate(i32 %a, i32 %b, i32 %c) {
> >   %1 = add i32 %a, %c
> >   call void @foo(i32 %1)
> >   %2 = add i32 %b, %c
> >   %3 = add i32 %a, %2
> >   call void @foo(i32 %3)
> >   ret void
> > }
> >
> >
> > normal reassociate transforms this, IMHO, wrongly, into:
> >
> > define void @left_reassociate(i32 %a, i32 %b, i32 %c) {
> >   %1 = add i32 %c, %a
> >   call void @foo(i32 %1)
> >   %2 = add i32 %b, %a
> >   %3 = add i32 %2, %c
> >   call void @foo(i32 %3)
> >   ret void
> > }
> >
> >
> > This is because for the first expression:
> >
> > RAIn: add i32 [ %a, #3] [ %c, #5]
> > RAOut: add i32 [ %c, #5] [ %a, #3]
> >
> >
> > and
> > for the second:
> > RAIn: add i32 [ %a, #3] [ %b, #4] [ %c, #5]
> > RAOut: add i32 [ %c, #5] [ %b, #4] [ %a, #3]
> >
> >
> > This makes it transform the first into add c, a
> > and the second into
> > %1 = add b, a
> > add c, %1
> >
> >
> > This is caused by these arguments having different ranks (because they
> > are function arguments), and it not respecting the same order it has
> > already chosen once.
> >
> >
> > This is, IMHO, pretty clearly a bug.
> >
> > It screws up right_reassociate, no_reassociate, and conditional tests
> > for the same reason.
> >
> > If you fix this, i expect you will find a lot less use cases for
> > nary-reassociate.
> >
> >
> > The simple way to fix this is to mark which operands it's paired
> > together in the past, and always try to pair the same ones together if
> > it can, regardless of rank.
>
> Or simply adjust the ranks of paired operands to be the same and
> different from all other ranks
>
> (BBrank is << 16, so you should have room do to this)
>
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