[NewGVN] Fold equivalent freeze instructions

[foad]

Based on D75398.

[nikic]

LGTM

[foad]

Rebase on fixed test case.

[aqjune]

To check, does NewGVN fully replace one instruction with another, it they are considered equivalent?

This is to ensure that partial replacements like this doesn't happen:

a = freeze(x)
b = freeze(x)
use(a)
use(b)
use(b)
=/=>
use(a)
use(a)
use(b) // this is wrong; this also must be use(a).

b in this example must be fully replaced with a to be correct.

[foad]

To check, does NewGVN fully replace one instruction with another, it they are considered equivalent?

I am not 100% sure. Do we have any NewGVN experts?

[kmitropoulou]

In D152529#4409089, @foad wrote:

To check, does NewGVN fully replace one instruction with another, it they are considered equivalent?

I am not 100% sure. Do we have any NewGVN experts?

The operands of the two freeze instructions are the same. So, the two freeze instructions are equivalent. Therefore, all the uses of b will be replaced with a.

[foad]

In D152529#4409191, @kmitropoulou wrote:

In D152529#4409089, @foad wrote:

To check, does NewGVN fully replace one instruction with another, it they are considered equivalent?

I am not 100% sure. Do we have any NewGVN experts?

The operands of the two freeze instructions are the same. So, the two freeze instructions are equivalent. Therefore, all the uses of b will be replaced with a.

Thanks! That's good enough for me.

[aqjune]

Thanks for a reply!

llvm/lib/Transforms/Scalar/NewGVN.cpp

else if (isa<Argument>(V) || isa<GlobalVariable>(V)) {
     case Instruction::Store:
       E = performSymbolicStoreEvaluation(I);
       break;
     case Instruction::Load:
       E = performSymbolicLoadEvaluation(I);
       break;
     case Instruction::BitCast:
     case Instruction::AddrSpaceCast:
+    case Instruction::Freeze:
       return createExpression(I);
       break;
     case Instruction::ICmp:
     case Instruction::FCmp:
       return performSymbolicCmpEvaluation(I);
       break;
     case Instruction::FNeg:
     case Instruction::Add:

llvm/test/Transforms/NewGVN/freeze.ll

 ; NOTE: Assertions have been autogenerated by utils/update_test_checks.py
 ; RUN: opt < %s -passes=newgvn -S | FileCheck %s
 
 define i1 @f(i1 %a) {
 ; CHECK-LABEL: @f(
 ; CHECK-NEXT:    [[B:%.*]] = freeze i1 [[A:%.*]]
-; CHECK-NEXT:    [[C:%.*]] = freeze i1 [[A]]
-; CHECK-NEXT:    [[D:%.*]] = and i1 [[B]], [[C]]
-; CHECK-NEXT:    ret i1 [[D]]
+; CHECK-NEXT:    ret i1 [[B]]
 ;
   %b = freeze i1 %a
   %c = freeze i1 %a
   %d = and i1 %b, %c
   ret i1 %d
 }
 
 define void @f_multipleuses(i1 %a) {
 ; CHECK-LABEL: @f_multipleuses(
 ; CHECK-NEXT:    [[B:%.*]] = freeze i1 [[A:%.*]]
-; CHECK-NEXT:    [[C:%.*]] = freeze i1 [[A]]
 ; CHECK-NEXT:    call void @use1(i1 [[B]])
-; CHECK-NEXT:    call void @use1(i1 [[C]])
-; CHECK-NEXT:    call void @use1(i1 [[C]])
+; CHECK-NEXT:    call void @use1(i1 [[B]])
+; CHECK-NEXT:    call void @use1(i1 [[B]])
 ; CHECK-NEXT:    ret void
 ;
   %b = freeze i1 %a
   %c = freeze i1 %a
   call void @use1(i1 %b)
   call void @use1(i1 %c)
   call void @use1(i1 %c)
   ret void