[Webkit-unassigned] [Bug 151335] B3 Patchpoint and Check opcodes should be able to specify Air::Arg::LateUse for their stackmap

[bugzilla-daemon at webkit.org]

https://bugs.webkit.org/show_bug.cgi?id=151335

Filip Pizlo <fpizlo at apple.com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
            Summary|B3::Patchpoints should be   |B3 Patchpoint and Check
                   |able to specify             |opcodes should be able to
                   |Air::Arg::LateUse           |specify Air::Arg::LateUse
                   |                            |for their stackmap

--- Comment #1 from Filip Pizlo <fpizlo at apple.com> ---
This is a bit tricky.  Currently, we have five opcodes that have stackmaps:

Patchpoint
Check
CheckAdd
CheckSub
CheckMul

CheckMul already uses LateUse for its stackmap, because the multiply cannot be undone.  Hence we need to ensure that if we want to recover any of the input values, they cannot overlap with the result.

The other opcodes currently have no way of specifying LateUse for their stackmaps.  It's tricky because currently the machinery for forcing LateUse in CheckMul relies on making the *entire* stackmap into LateUse.  Specifying LateUse on the entire stackmap is super dangerous because it also implies cold use, which is incompatible with constraints like SomeRegister.  What we really want is to specify LateUse on only some of the stackmap arguments. Ideally, there would be some way of saying LateUse in the ValueRep input constraints.  For example, we could have a ValueRep::Late, which is like Any but means LateUse.

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