News for hackers. Stuff that matters.

A comment on GCC 7 Release Series – Changes, New Features, and Fixes. Parent comment

Re:GCC 7 Release Series – Changes, New Features, and Fixes by mindleyhilner on Sunday April 09, 2017 @01:09PM (#14073239)
The point of hoisting towards entry is to reduce code size, as the changelog indicates. It's safe to hoist the expression from program points P_{0..i..n-1} to some point Q that dominates all P_i if the expression is available at each P_i. That reduces the number of occurrences by n - 1. It's a basic corollary of PRE.

This is identical to what LLVM's load-store motion does for loads, except for all expressions and not just loads.

[ Parent | on HN ]
Re:GCC 7 Release Series – Changes, New Features, and Fixes by DannyBee on Sunday April 09, 2017 @01:10PM (#14073245) 

  This is identical to what LLVM's load-store motion does for 
  loads, except for all expressions and not just loads.
It's actually not. It's what GVNHoist does, but not MLSM. MLSM only handles diamonds.

In any case, because the GCC implementation is written on top of a sane PRE infrastructure, it is like 50 lines of code to do this :)

[ Parent | on HN ]
Re:GCC 7 Release Series – Changes, New Features, and Fixes by mindleyhilner on Sunday April 09, 2017 @01:14PM (#14073273) 

  It's actually not. It's what GVNHoist does, but not MLSM. MLSM only handles diamonds.
Fine, it's what MLSM aspires to: http://llvm-cs.pcc.me.uk/lib/Transforms/Scalar/MergedLoadSto... (:
[ Parent | on HN ]
Re:GCC 7 Release Series – Changes, New Features, and Fixes by DannyBee on Sunday April 09, 2017 @04:53PM (#14074303)
Writing "TODO: generalize to other regions" in a crazy N^3/N^4 approach to merging load/stores is like writing "TODO: solve prime number conjecture" in somebody's hand-rolled two's complement addition code.

Maybe it happens, but it ain't gonna happen with anything like this code :)

[ Parent | on HN ]