[AMDGPU] Stop replacing amdgcn.ballot(1) with amdgcn.s.getreg(exec)

[foad]

Summary

Rationale:

• It does not enable any further IR simplifications.
• It does not improve the generated code since the isel lowering of ballot also has special cases for 0 and 1.
• getreg is "too powerful" since it can read from many different registers, so its intrinsic properties have to be set very conservatively.

There is also a correctness problem that getreg can read from exec but
it is currently not marked as convergent.

[arsenm]

There is also a correctness problem that getreg can read from exec but it is currently not marked as convergent.

You can mark the call sites convergent, which this is doing. We still have __builtin_amdgcn_read_exec emitting read_register, but I was thinking we should move them to ballot(true)

[foad]

You can mark the call sites convergent, which this is doing.

Hmm, I had not noticed that. I will take another look at the correctness problem that I thought I was seeing.

[foad]

In D153047#4425377, @foad wrote:

You can mark the call sites convergent, which this is doing.

Hmm, I had not noticed that. I will take another look at the correctness problem that I thought I was seeing.

D153151 should fix that, but I still plan to push this patch.

llvm/lib/Target/AMDGPU/AMDGPUInstCombineIntrinsic.cpp

case Intrinsic::amdgcn_fcmp: {
     break;
   }
   case Intrinsic::amdgcn_ballot: {
     if (auto *Src = dyn_cast<ConstantInt>(II.getArgOperand(0))) {
       if (Src->isZero()) {
         // amdgcn.ballot(i1 0) is zero.
         return IC.replaceInstUsesWith(II, Constant::getNullValue(II.getType()));
       }
-
-      if (Src->isOne()) {
-        // amdgcn.ballot(i1 1) is exec.
-        const char *RegName = "exec";
-        if (II.getType()->isIntegerTy(32))
-          RegName = "exec_lo";
-        else if (!II.getType()->isIntegerTy(64))
-          break;
-
-        Function *NewF = Intrinsic::getDeclaration(
-            II.getModule(), Intrinsic::read_register, II.getType());
-        Metadata *MDArgs[] = {MDString::get(II.getContext(), RegName)};
-        MDNode *MD = MDNode::get(II.getContext(), MDArgs);
-        Value *Args[] = {MetadataAsValue::get(II.getContext(), MD)};
-        CallInst *NewCall = IC.Builder.CreateCall(NewF, Args);
-        NewCall->addFnAttr(Attribute::Convergent);
-        NewCall->takeName(&II);
-        return IC.replaceInstUsesWith(II, NewCall);
-      }
     }
     break;
   }
   case Intrinsic::amdgcn_wqm_vote: {
     // wqm_vote is identity when the argument is constant.
     if (!isa<Constant>(II.getArgOperand(0)))
       break;
 

llvm/test/Transforms/InstCombine/AMDGPU/amdgcn-intrinsics.ll

 ; CHECK-NEXT:    ret i64 0
 ;
   %b = call i64 @llvm.amdgcn.ballot.i64(i1 0)
   ret i64 %b
 }
 
 define i64 @ballot_one_64() {
 ; CHECK-LABEL: @ballot_one_64(
-; CHECK-NEXT:    [[B:%.*]] = call i64 @llvm.read_register.i64(metadata [[META0]]) #[[ATTR17]]
+; CHECK-NEXT:    [[B:%.*]] = call i64 @llvm.amdgcn.ballot.i64(i1 true)
 ; CHECK-NEXT:    ret i64 [[B]]
 ;
   %b = call i64 @llvm.amdgcn.ballot.i64(i1 1)
   ret i64 %b
 }
 
 define i32 @ballot_nocombine_32(i1 %i) {
 ; CHECK-LABEL: @ballot_nocombine_32(

 ; CHECK-NEXT:    ret i32 0
 ;
   %b = call i32 @llvm.amdgcn.ballot.i32(i1 0)
   ret i32 %b
 }
 
 define i32 @ballot_one_32() {
 ; CHECK-LABEL: @ballot_one_32(
-; CHECK-NEXT:    [[B:%.*]] = call i32 @llvm.read_register.i32(metadata [[META1:![0-9]+]]) #[[ATTR17]]
+; CHECK-NEXT:    [[B:%.*]] = call i32 @llvm.amdgcn.ballot.i32(i1 true)
 ; CHECK-NEXT:    ret i32 [[B]]
 ;
   %b = call i32 @llvm.amdgcn.ballot.i32(i1 1)
   ret i32 %b
 }
 
 ; --------------------------------------------------------------------
 ; llvm.amdgcn.wqm.vote