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Just to make sure we're on the same page, let me try to rephrase
your problem:<br>
"After the initial call is patched out of existence, the register
spilling which was reasonable for the initial call becomes a
performance liability for the patched in assembly."<br>
<br>
Is that a correct restatement?<br>
<br>
If so, I'd suggest you look into not using the anyregcc convention.
My understand is that that convention is intended for calls where
values are desired in registers over the call. This is not what you
want.<br>
<br>
If you used the standard C convention, the calling convention would
place the first X (where X is 6 I think) arguments in registers and
the rest on the stack. If you pad your actual arguments with undef
such that the arguments that are used by your patch mechanism are
after that threshold, you might see what you're looking for.
However, I suspect you'd still see a move from the stack to a
register, and then back to a different slot due the calling
convention adjustments.<br>
<br>
What I think you really want is a parameter attribute that we don't
have. You want to be able to tag a particular parameter as being
"anylocation" (analogous to the anyreg of anyregcc). In concept, a
"anystack" notion is similar to what we do for gc.statepoints (and,
I think, excess arguments on patchpoints?). That's still not quite
as strong as what you want though...<br>
<br>
If you do decide to pursue the "anylocation" parameter attribute,
definitely keep me informed. This is analogous to something we want
for gc.statepoints as well. <br>
<br>
p.s. Without knowing what DB actually is, it really looks like you
might be able to accomplish the same thing using the "ID" field of
the patchpoint. Have you looked into that?<br>
<br>
Philip<br>
<br>
<br>
<br>
<div class="moz-cite-prefix">On 02/19/2015 04:46 AM, Frej Drejhammar
wrote:<br>
</div>
<blockquote cite="mid:tz8lhjuhxvw.fsf@sics.se" type="cite">
<pre wrap="">Hi All,
I am observing something i suspect is a misbehaviour of the register
allocator which impacts the performance of patchpoints. This occurs in
the context of an abstract machine which in some places uses inline
caches. The problematic code looks like this:
entry: ; Initialize the abstract machine
%db = call create_big_seldom_used_database()
; do a lot of things which increases register pressure and spills %db
br label %main_execution_loop;
main_execution_loop:
; We do instruction dispatch here
...
opcode_a:
%name0 = ...
; Use the database to look up %name0 and then overwrite the patchpoint
; with a direct call
tail call anyregcc void (i64, i32, i8*, i32, ...)*
@llvm.experimental.patchpoint.void(i64 4711, i32 16, @lookup_and_patch, i32 0,
some_type %name0, some_type %db)
...
%name1 = ...
; Use the database to look up %name1 and then overwrite the patchpoint
; with a direct call
tail call anyregcc void (i64, i32, i8*, i32, ...)*
@llvm.experimental.patchpoint.void(i64 4711, i32 16, @lookup_and_patch, i32 0,
some_type %name1, some_type %db)
...
br label %main_execution_loop;
If I run this through llc (for x86_64) I will frequently see, especially
if I have two cache lookups in the same basic block or low register
pressure, that %db is loaded from the stack and into a register. The
generated code looks like this:
reload %db into reg0
; %name0 is in reg1
call lookup_and_patch(reg1, reg0)
; shadow
; %name1 is in reg2
call lookup_and_patch(reg1, reg0)
; shadow
This is a performance problem as, although the calls to
lookup_and_patch() are overwritten, we will always pay for the, now
useless, load of %db into reg0. If I wanted the arguments to
lookup_and_patch() in registers I would not have used the anyregcc
calling convention. In this toy example lookup_and_patch() only refers a
single variable, but in my real application it uses multiple values
(most of them spilled) and the slowdown is quite noticeable (overheads
of up to 600% for some opcodes).
To me this looks like the register allocator is too eager to load values
which are only used by anyregcc patchpoints into registers, or is this
the intended behavior of anyregcc patchpoints?
I would be grateful for suggestions of how I could modify the register
allocator (RAGreedy) to avoid reloading values when they are only used
by instructions which are anyregcc patchpoints. During the last two
weeks I have made a couple unsuccessful attempts at that and could
really use some pointers from someone who understands it.
Attached is a the smallest example I have managed to find which shows
the problem.
Regards,
--Frej
</pre>
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</pre>
</blockquote>
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