reames added a comment.
Here are the minutes from our phone call a few minutes ago.
Attendees: Andy Kaylor, Craig Topper, Annita Zhang, Tom Stellard, Chandler
Carruth, Fedor Sergeev, Philip Reames, Yuanake Luo
Status Summary
==============
Performance data has been posted to llvm-dev. We had a side discussion about
nop encoding, and it was mentioned these numbers were collected from runs
targeting skylake (i.e. not generic x86). This is similar to the result we
(Azul) have collected and shared summaries of previously.
GNU patch has landed Friday - this mostly fixes assembler command line.
Discussion on Approach
======================
Three major options debated:
Assembler only - as in the current patch, assembler does all work, only
command line flag
Explicit Directive only - as proposed in my alternate patch, compiler decides
exactly what instructions get aligned
Region based directives - as proposed in James' last comment on review,
directives enable and disable auto-padding in assembler
Use cases identified:
compiler users
important than assembler is self contained (i.e. don't have to know
compiler options for reproduceability)
inline assembly looks a lot like assembler users
legacy assembler
important that existing assembly works unmodified
assembler users
"try it and see" model vs selective enable vs selective disable
likely need to support all three
Consensus was that the region based directives met use cases the best. In
particular, desire to be able to overrule default (for say, inline assembly or
a JITs patchability assumptions) and then restore default. Default assembler
behavior remains unchanged.
Stawman syntax proposal
.align_branch_boundary disable/default
.align_branch_boundary enable N, instructions (fused, jcc, jmp, etc..)
We need to ensure a consensus on syntax is shared w/gnu. Annita agreed to
coordinate this.
Compiler would essentially just wrap generated assembly in directives.
Issue noticed while writing this up: proposed syntax assumes a default has been
set, but doesn't give a way to set one. This would seem to break the desired
reproducibility property for compiled code. Revision needed.
Push/Pop semantics were suggested at one point, but were thought to be
non-idiomatic?
Other Topics
============
We very quickly discussed nop vs prefix performance. There was a clear
consensus in starting with nop only patch and evolving later as needed.
Next Steps
==========
Annita will refresh current patch with two key changes. 1) Drop prefix support
and simplify and 2) drop clang driver support for now. Desire is to minimize
cycle time before next iteration so that feedback on approach can be given
while reviewers are still around.
Philip will prototype directive parsing support. Annita and Yuo (??) to handle
coordination on syntax.
Suggested patch split:
(current patch) command line option to set default, nop only version
w/cleaned up code as much as possible
assembler directive support (draft by Philip in parallel)
(future) compiler patch to wrap by default
Side note to Annita: For you to remove "hard code", you'll have to have a
placeholder for the enable/disable interface. That should probably be split
and rebased in my patch.
CHANGES SINCE LAST ACTION
https://reviews.llvm.org/D70157/new/
https://reviews.llvm.org/D70157
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