[all-commits] [llvm/llvm-project] ecb9d8: [LICM] Hoist LOAD without sinking the STORE

[Djordje Todorovic via All-commits]

  Branch: refs/heads/main
  Home:   https://github.com/llvm/llvm-project
  Commit: ecb9d8e4e3c4623c2edcd2c50727103927d31508
      https://github.com/llvm/llvm-project/commit/ecb9d8e4e3c4623c2edcd2c50727103927d31508
  Author: Djordje Todorovic <djordje.todorovic at syrmia.com>
  Date:   2021-12-01 (Wed, 01 Dec 2021)

  Changed paths:
    M llvm/include/llvm/Transforms/Utils/SSAUpdater.h
    M llvm/lib/Transforms/Scalar/LICM.cpp
    M llvm/lib/Transforms/Utils/SSAUpdater.cpp
    M llvm/test/Transforms/InstMerge/st_sink_bugfix_22613.ll
    A llvm/test/Transforms/LICM/hoist-load-without-store.ll
    M llvm/test/Transforms/LICM/promote-capture.ll
    M llvm/test/Transforms/LICM/scalar-promote-memmodel.ll
    M llvm/test/Transforms/LICM/scalar-promote.ll

  Log Message:
  -----------
  [LICM] Hoist LOAD without sinking the STORE

When doing load/store promotion within LICM, if we
cannot prove that it is safe to sink the store we won't
hoist the load, even though we can prove the load could
be dereferenced and moved outside the loop. This patch
implements the load promotion by moving it in the loop
preheader by inserting proper PHI in the loop. The store
is kept as is in the loop. By doing this, we avoid doing
the load from a memory location in each iteration.

Please consider this small example:

loop {
  var = *ptr;
  if (var) break;
  *ptr= var + 1;
}
After this patch, it will be:

var0 = *ptr;
loop {
  var1 = phi (var0, var2);
  if (var1) break;
  var2 = var1 + 1;
  *ptr = var2;
}
This addresses some problems from [0].

[0] https://bugs.llvm.org/show_bug.cgi?id=51193

Differential revision: https://reviews.llvm.org/D113289