[all-commits] [llvm/llvm-project] d5244f: [WebAssembly] Use SSAUpdaterBulk in LowerEmscripte...

[Heejin Ahn via All-commits]

  Branch: refs/heads/main
  Home:   https://github.com/llvm/llvm-project
  Commit: d5244fb16070068fc1dfae5804ab8ff86d25deb9
      https://github.com/llvm/llvm-project/commit/d5244fb16070068fc1dfae5804ab8ff86d25deb9
  Author: Heejin Ahn <aheejin at gmail.com>
  Date:   2021-08-24 (Tue, 24 Aug 2021)

  Changed paths:
    M llvm/lib/Target/WebAssembly/WebAssemblyLowerEmscriptenEHSjLj.cpp
    M llvm/test/CodeGen/WebAssembly/lower-em-sjlj.ll

  Log Message:
  -----------
  [WebAssembly] Use SSAUpdaterBulk in LowerEmscriptenSjLj

We update SSA in two steps in Emscripten SjLj:
1. Rewrite uses of `setjmpTable` and `setjmpTableSize` variables and
   place `phi`s where necessary, which are updated where we call
   `saveSetjmp`.
2. Do a whole function level SSA update for all variables, because we
   split BBs where `setjmp` is called and there are possibly variable
   uses that are not dominated by a def.
   (See https://github.com/llvm/llvm-project/blob/955b91c19c00ed4c917559a5d66d14c669dde2e3/llvm/lib/Target/WebAssembly/WebAssemblyLowerEmscriptenEHSjLj.cpp#L1314-L1324)

We have been using `SSAUpdater` to do this, but `SSAUpdaterBulk` class
was added after this pass was first created, and for the step 2 it looks
like a better alternative with a possible performance benefit. Not sure
the author is aware of it, but `SSAUpdaterBulk` seems to have a
limitation: it cannot handle a use within the same BB as a def but
before it. For example:
```
... = %a + 1
%a = foo();
```
or
```
%a = %a + 1
```
The uses `%a` in RHS should be rewritten with another SSA variable of
`%a`, most likely one generated from a `phi`. But `SSAUpdaterBulk`
thinks all uses of `%a` are below the def of `%a` within the same BB.
(`SSAUpdater` has two different functions of rewriting because of this:
`RewriteUse` and `RewriteUseAfterInsertions`.) This doesn't affect our
usage in the step 2 because that deals with possibly non-dominated uses
by defs after block splitting. But it does in the step 1, which still
uses `SSAUpdater`.

But this CL also simplifies the step 1 by using `make_early_inc_range`,
removing the need to advance the iterator before rewriting a use.

This is NFC; the test changes are just the order of PHI nodes.

Reviewed By: dschuff

Differential Revision: https://reviews.llvm.org/D108583