[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

Author: aqjune

llvm/lib/CodeGen/CodeGenPrepare.cpp

@@ -6131,14 +6131,16 @@ 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:
   //       br label %select.end
   //    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());

llvm/test/CodeGen/ARM/2012-08-30-select.ll

@@ -2,9 +2,9 @@
 ; rdar://12201387
 
 ;CHECK-LABEL: select_s_v_v:
+;CHECK: vmov.i32
 ;CHECK: vmov
 ;CHECK-NEXT: vmov
-;CHECK: vmov.i32
 ;CHECK: bx
 define <16 x i8> @select_s_v_v(<16 x i8> %vec, i32 %avail) {
 entry:

llvm/test/Transforms/CodeGenPrepare/X86/optimizeSelect-DT.ll

@@ -7,7 +7,8 @@ target triple = "x86_64-unknown-linux-gnu"
 define i1 @PR41004(i32 %x, i32 %y, i32 %t1) {
 ; CHECK-LABEL: @PR41004(
 ; CHECK-NEXT:  entry:
-; CHECK-NEXT:    [[T0:%.*]] = icmp eq i32 [[Y:%.*]], 1
+; CHECK-NEXT:    [[MUL_FR:%.*]] = freeze i32 [[Y:%.*]]
+; CHECK-NEXT:    [[T0:%.*]] = icmp eq i32 [[MUL_FR]], 1
 ; CHECK-NEXT:    br i1 [[T0]], label [[SELECT_TRUE_SINK:%.*]], label [[SELECT_END:%.*]]
 ; CHECK:       select.true.sink:
 ; CHECK-NEXT:    [[REM:%.*]] = srem i32 [[X:%.*]], 2

llvm/test/Transforms/CodeGenPrepare/X86/select.ll

@@ -27,7 +27,8 @@ entry:
 define float @fdiv_true_sink(float %a, float %b) {
 ; CHECK-LABEL: @fdiv_true_sink(
 ; CHECK-NEXT:  entry:
-; CHECK-NEXT:    [[CMP:%.*]] = fcmp ogt float [[A:%.*]], 1.000000e+00
+; CHECK-NEXT:    [[SEL_FR:%.*]] = freeze float [[A:%.*]]
+; CHECK-NEXT:    [[CMP:%.*]] = fcmp ogt float [[SEL_FR]], 1.000000e+00
 ; CHECK-NEXT:    br i1 [[CMP]], label [[SELECT_TRUE_SINK:%.*]], label [[SELECT_END:%.*]]
 ; CHECK:       select.true.sink:
 ; CHECK-NEXT:    [[DIV:%.*]] = fdiv float [[A]], [[B:%.*]]
@@ -38,7 +39,8 @@ define float @fdiv_true_sink(float %a, float %b) {
 ;
 ; DEBUG-LABEL: @fdiv_true_sink(
 ; DEBUG-NEXT:  entry:
-; DEBUG-NEXT:    [[CMP:%.*]] = fcmp ogt float [[A:%.*]], 1.000000e+00
+; DEBUG-NEXT:    [[SEL_FR:%.*]] = freeze float [[A:%.*]]
+; DEBUG-NEXT:    [[CMP:%.*]] = fcmp ogt float [[SEL_FR]], 1.000000e+00, !dbg !24
 ; DEBUG-NEXT:    call void @llvm.dbg.value(metadata i1 [[CMP]]
 ; DEBUG-NEXT:    br i1 [[CMP]], label [[SELECT_TRUE_SINK:%.*]], label [[SELECT_END:%.*]], !dbg
 ; DEBUG:       select.true.sink:
@@ -60,7 +62,8 @@ entry:
 define float @fdiv_false_sink(float %a, float %b) {
 ; CHECK-LABEL: @fdiv_false_sink(
 ; CHECK-NEXT:  entry:
-; CHECK-NEXT:    [[CMP:%.*]] = fcmp ogt float [[A:%.*]], 3.000000e+00
+; CHECK-NEXT:    [[SEL_FR:%.*]] = freeze float [[A:%.*]]
+; CHECK-NEXT:    [[CMP:%.*]] = fcmp ogt float [[SEL_FR]], 3.000000e+00
 ; CHECK-NEXT:    br i1 [[CMP]], label [[SELECT_END:%.*]], label [[SELECT_FALSE_SINK:%.*]]
 ; CHECK:       select.false.sink:
 ; CHECK-NEXT:    [[DIV:%.*]] = fdiv float [[A]], [[B:%.*]]
@@ -71,7 +74,8 @@ define float @fdiv_false_sink(float %a, float %b) {
 ;
 ; DEBUG-LABEL: @fdiv_false_sink(
 ; DEBUG-NEXT:  entry:
-; DEBUG-NEXT:    [[CMP:%.*]] = fcmp ogt float [[A:%.*]], 3.000000e+00
+; DEBUG-NEXT:    [[SEL_FR:%.*]] = freeze float [[A:%.*]]
+; DEBUG-NEXT:    [[CMP:%.*]] = fcmp ogt float [[SEL_FR]], 3.000000e+00, !dbg !33
 ; DEBUG-NEXT:    call void @llvm.dbg.value(metadata i1 [[CMP]]
 ; DEBUG-NEXT:    br i1 [[CMP]], label [[SELECT_END:%.*]], label [[SELECT_FALSE_SINK:%.*]], !dbg
 ; DEBUG:       select.false.sink:
@@ -93,7 +97,8 @@ entry:
 define float @fdiv_both_sink(float %a, float %b) {
 ; CHECK-LABEL: @fdiv_both_sink(
 ; CHECK-NEXT:  entry:
-; CHECK-NEXT:    [[CMP:%.*]] = fcmp ogt float [[A:%.*]], 5.000000e+00
+; CHECK-NEXT:    [[SEL_FR:%.*]] = freeze float [[A:%.*]]
+; CHECK-NEXT:    [[CMP:%.*]] = fcmp ogt float [[SEL_FR]], 5.000000e+00
 ; CHECK-NEXT:    br i1 [[CMP]], label [[SELECT_TRUE_SINK:%.*]], label [[SELECT_FALSE_SINK:%.*]]
 ; CHECK:       select.true.sink:
 ; CHECK-NEXT:    [[DIV1:%.*]] = fdiv float [[A]], [[B:%.*]]