[LLVMdev] MI bundle liveness attributes

[Sergei Larin]

Hello everyone,

  Let me (re)present a question that might have previously been discussed,
but did not result in any code (AFIK).

  How do we represent a _conditional_ assignment (def) in a bundle MI?

  More contents - currently we expose internal def/use/kill information to a
bundle header - something like this:


BUNDLE %PC<imp-def>, %R0<imp-def>, %P0<imp-use,kill>, %R16<imp-use>
   * %R0<def> = LDriuh_cdnNotPt %P0<kill,internal>, %R16, 0;
   * %P0<def> = CMPEQri %R16, 0;

  Here CMPEQri is a compare to a predicate register instruction, and
LDriuh_cdnNotPt is a _conditional_ load, which might or might not
Take place based on the outcome of the compare... As such R0 might or might
not be defined in this bundle, which obviously changes the liveness update
process.

  My question, do we need another attribute along with isImplicit and
isEarlyClobber etc. to designate a conditional def? Furthermore, depending
on architectural details we well might have a conditional use as well... and
what about the individual (unbundled) def/use? Should this:

%R0<def> = LDriuh_cdnNotPt %P0<kill,internal>, %R16, 0;

...become this:

%R0<def-cond> = LDriuh_cdnNotPt %P0<kill,internal>, %R16, 0;

or even:

%R0<def-cond> = LDriuh_cdnNotPt %P0<kill,internal>, %R16<use-cond>, 0;

  So, if I am missing something in current implementation or an ongoing
discussions (and that is entirely possible since I am just back after
vacation), please let me know how to achieve this functionality, but if this
is something missing in implementation, let's discuss how do we want to
realize it.

  Thanks.

Sergei Larin

--
Qualcomm Innovation Center, Inc. is a member of Code Aurora Forum.