[all-commits] [llvm/llvm-project] e338d0: [SimplifyCFG] Fix SimplifyBranchOnICmpChain to be ...

[Hyeongyu Kim via All-commits]

  Branch: refs/heads/main
  Home:   https://github.com/llvm/llvm-project
  Commit: e338d08ae609bf07b1f43ad15a6488e7c302800b
      https://github.com/llvm/llvm-project/commit/e338d08ae609bf07b1f43ad15a6488e7c302800b
  Author: hyeongyu kim <gusrb406 at snu.ac.kr>
  Date:   2021-07-13 (Tue, 13 Jul 2021)

  Changed paths:
    M llvm/lib/Transforms/Utils/SimplifyCFG.cpp
    M llvm/test/Transforms/SimplifyCFG/switch_create-custom-dl.ll
    M llvm/test/Transforms/SimplifyCFG/switch_create.ll
    M llvm/test/Transforms/SimplifyCFG/switch_msan.ll

  Log Message:
  -----------
  [SimplifyCFG] Fix SimplifyBranchOnICmpChain to be undef/poison safe.

This patch fixes the problem of SimplifyBranchOnICmpChain that occurs
when extra values are Undef or poison.

Suppose the %mode is 51 and the %Cond is poison, and let's look at the
case below.
```
	%A = icmp ne i32 %mode, 0
	%B = icmp ne i32 %mode, 51
	%C = select i1 %A, i1 %B, i1 false
	%D = select i1 %C, i1 %Cond, i1 false
	br i1 %D, label %T, label %F
=>
	br i1 %Cond, label %switch.early.test, label %F
switch.early.test:
	switch i32 %mode, label %T [
		i32 51, label %F
		i32 0, label %F
	]
```
incorrectness: https://alive2.llvm.org/ce/z/BWScX

Code before transformation will not raise UB because %C and %D is false,
and it will not use %Cond. But after transformation, %Cond is being used
immediately, and it will raise UB.

This problem can be solved by adding freeze instruction.

correctness: https://alive2.llvm.org/ce/z/x9x4oY

Reviewed By: nikic

Differential Revision: https://reviews.llvm.org/D104569