[all-commits] [llvm/llvm-project] 60dd01: Revert "[SimplifyCFG] performBranchToCommonDestFol...

[Roman Lebedev via All-commits]

  Branch: refs/heads/main
  Home:   https://github.com/llvm/llvm-project
  Commit: 60dd0121c92e93a33cf39b5f7006923ac9e4f127
      https://github.com/llvm/llvm-project/commit/60dd0121c92e93a33cf39b5f7006923ac9e4f127
  Author: Roman Lebedev <lebedev.ri at gmail.com>
  Date:   2021-08-15 (Sun, 15 Aug 2021)

  Changed paths:
    M llvm/lib/Transforms/Utils/SimplifyCFG.cpp

  Log Message:
  -----------
  Revert "[SimplifyCFG] performBranchToCommonDestFolding(): form block-closed SSA form before cloning instructions (PR51125)"

Forgot to stage the test change.

This reverts commit 78af5cb213b2f9fe3f47bf23947f14ac07024155.


  Commit: 3d9beefc7d713ad8462d92427ccd17b9532ce904
      https://github.com/llvm/llvm-project/commit/3d9beefc7d713ad8462d92427ccd17b9532ce904
  Author: Roman Lebedev <lebedev.ri at gmail.com>
  Date:   2021-08-15 (Sun, 15 Aug 2021)

  Changed paths:
    M llvm/lib/Transforms/Utils/SimplifyCFG.cpp
    M llvm/test/Transforms/SimplifyCFG/fold-branch-to-common-dest.ll

  Log Message:
  -----------
  Reland [SimplifyCFG] performBranchToCommonDestFolding(): form block-closed SSA form before cloning instructions (PR51125)

... with test change this time.

LLVM IR SSA form is "implicit" in `@pr51125`. While is a valid LLVM IR,
and does not require any PHI nodes, that completely breaks the further logic
in `CloneInstructionsIntoPredecessorBlockAndUpdateSSAUses()`
that updates the live-out uses of the bonus instructions.

What i believe we need to do, is to first make the SSA form explicit,
by inserting tautological PHI nodes, and rewriting the offending uses.

```
$ /builddirs/llvm-project/build-Clang12/bin/opt -load /repositories/alive2/build-Clang-release/tv/tv.so -load-pass-plugin /repositories/alive2/build-Clang-release/tv/tv.so -tv -simplifycfg -simplifycfg-require-and-preserve-domtree=1 -bonus-inst-threshold=10 -tv -o /dev/null /tmp/test.ll

----------------------------------------
@global_pr51125 = global 4 bytes, align 4

define i32 @pr51125() {
%entry:
  br label %L

%L:
  %ld = load i32, * @global_pr51125, align 4
  %iszero = icmp eq i32 %ld, 0
  br i1 %iszero, label %exit, label %L2

%L2:
  store i32 4294967295, * @global_pr51125, align 4
  %cmp = icmp eq i32 %ld, 4294967295
  br i1 %cmp, label %L, label %exit

%exit:
  %r = phi i32 [ %ld, %L2 ], [ %ld, %L ]
  ret i32 %r
}
=>
@global_pr51125 = global 4 bytes, align 4

define i32 @pr51125() {
%entry:
  %ld.old = load i32, * @global_pr51125, align 4
  %iszero.old = icmp eq i32 %ld.old, 0
  br i1 %iszero.old, label %exit, label %L2

%L2:
  %ld2 = phi i32 [ %ld.old, %entry ], [ %ld, %L2 ]
  store i32 4294967295, * @global_pr51125, align 4
  %cmp = icmp ne i32 %ld2, 4294967295
  %ld = load i32, * @global_pr51125, align 4
  %iszero = icmp eq i32 %ld, 0
  %or.cond = select i1 %cmp, i1 1, i1 %iszero
  br i1 %or.cond, label %exit, label %L2

%exit:
  %ld1 = phi i32 [ poison, %L2 ], [ %ld.old, %entry ]
  %r = phi i32 [ %ld2, %L2 ], [ %ld.old, %entry ]
  ret i32 %r
}
Transformation seems to be correct!

```

Fixes https://bugs.llvm.org/show_bug.cgi?id=51125

Reviewed By: nikic

Differential Revision: https://reviews.llvm.org/D106317


Compare: https://github.com/llvm/llvm-project/compare/78af5cb213b2...3d9beefc7d71