[CodeGenPrepare] Freeze condition when transforming select to br

Summary:
This is a simple fix for CodeGenPrepare that freezes branch condition when transforming select to branch.
If it is not freezed, instsimplify or the later pipeline can potentially exploit undefined behavior.

The diff shows optimized form becase D75859 and D76048 already made a few changes to CodeGenPrepare for optimizing freeze(cmp).

Reviewers: jdoerfert, spatel, lebedev.ri, efriedma

Reviewed By: lebedev.ri

Subscribers: hiraditya, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D76179

1 files changed, 6 insertions, 2 deletions

diff --git a/llvm/lib/CodeGen/CodeGenPrepare.cpp b/llvm/lib/CodeGen/CodeGenPrepare.cpp
index 373064d..fd55ef2 100644
--- a/llvm/lib/CodeGen/CodeGenPrepare.cpp
+++ b/llvm/lib/CodeGen/CodeGenPrepare.cpp
@@ -6131,7 +6131,8 @@ bool CodeGenPrepare::optimizeSelectInst(SelectInst *SI) {
   // Into:
   //    start:
   //       %cmp = cmp uge i32 %a, %b
-  //       br i1 %cmp, label %select.true, label %select.false
+  //       %cmp.frozen = freeze %cmp
+  //       br i1 %cmp.frozen, label %select.true, label %select.false
   //    select.true:
   //       br label %select.end
   //    select.false:
@@ -6139,6 +6140,7 @@ bool CodeGenPrepare::optimizeSelectInst(SelectInst *SI) {
   //    select.end:
   //       %sel = phi i32 [ %c, %select.true ], [ %d, %select.false ]
   //
+  // %cmp should be freezed, otherwise it may introduce undefined behavior.
   // In addition, we may sink instructions that produce %c or %d from
   // the entry block into the destination(s) of the new branch.
   // If the true or false blocks do not contain a sunken instruction, that
@@ -6217,7 +6219,9 @@ bool CodeGenPrepare::optimizeSelectInst(SelectInst *SI) {
     TT = TrueBlock;
     FT = FalseBlock;
   }
-  IRBuilder<>(SI).CreateCondBr(SI->getCondition(), TT, FT, SI);
+  IRBuilder<> IB(SI);
+  auto CondFr = IB.CreateFreeze(SI->getCondition(), SI->getName() + ".frozen");
+  IB.CreateCondBr(CondFr, TT, FT, SI);
 
   SmallPtrSet<const Instruction *, 2> INS;
   INS.insert(ASI.begin(), ASI.end());