[PATCH] D153047: [AMDGPU] Stop replacing amdgcn.ballot(1) with amdgcn.s.getreg(exec)

[Jay Foad via Phabricator via llvm-commits]

foad created this revision.
foad added reviewers: AMDGPU, sebastian-ne, nhaehnle.
Herald added subscribers: StephenFan, kerbowa, hiraditya, tpr, dstuttard, yaxunl, jvesely, kzhuravl, arsenm.
Herald added a project: All.
foad requested review of this revision.
Herald added subscribers: llvm-commits, wdng.
Herald added a project: LLVM.

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.


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D153047

Files:
  llvm/lib/Target/AMDGPU/AMDGPUInstCombineIntrinsic.cpp
  llvm/test/Transforms/InstCombine/AMDGPU/amdgcn-intrinsics.ll


Index: llvm/test/Transforms/InstCombine/AMDGPU/amdgcn-intrinsics.ll
===================================================================
--- llvm/test/Transforms/InstCombine/AMDGPU/amdgcn-intrinsics.ll
+++ llvm/test/Transforms/InstCombine/AMDGPU/amdgcn-intrinsics.ll
@@ -2582,7 +2582,7 @@
 
 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)
@@ -2608,7 +2608,7 @@
 
 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)
Index: llvm/lib/Target/AMDGPU/AMDGPUInstCombineIntrinsic.cpp
===================================================================
--- llvm/lib/Target/AMDGPU/AMDGPUInstCombineIntrinsic.cpp
+++ llvm/lib/Target/AMDGPU/AMDGPUInstCombineIntrinsic.cpp
@@ -908,25 +908,6 @@
         // 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;
   }


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