[PATCH] D32344: InstCombine/AMDGPU: Fix constant folding of llvm.amdgcn.{icmp, fcmp}

[Nicolai Hähnle via Phabricator via llvm-commits]

nhaehnle created this revision.
Herald added subscribers: t-tye, tpr, dstuttard, yaxunl, wdng, kzhuravl.

The return value of these intrinsics should always have 0 bits for
inactive threads. This means that when all arguments are constant
and the comparison evaluates to true, the intrinsic should return
the current exec mask.

Fixes some GL_ARB_shader_ballot tests.


https://reviews.llvm.org/D32344

Files:
  lib/Transforms/InstCombine/InstCombineCalls.cpp
  test/Transforms/InstCombine/amdgcn-intrinsics.ll


Index: test/Transforms/InstCombine/amdgcn-intrinsics.ll
===================================================================
--- test/Transforms/InstCombine/amdgcn-intrinsics.ll
+++ test/Transforms/InstCombine/amdgcn-intrinsics.ll
@@ -1259,7 +1259,7 @@
 }
 
 ; CHECK-LABEL: @icmp_constant_inputs_true(
-; CHECK: ret i64 -1
+; CHECK: %result = call i64 @llvm.read_register.i64(metadata !0) #4
 define i64 @icmp_constant_inputs_true() {
   %result = call i64 @llvm.amdgcn.icmp.i32(i32 9, i32 8, i32 34)
   ret i64 %result
@@ -1524,7 +1524,7 @@
 }
 
 ; CHECK-LABEL: @fcmp_constant_inputs_true(
-; CHECK: ret i64 -1
+; CHECK: %result = call i64 @llvm.read_register.i64(metadata !0) #4
 define i64 @fcmp_constant_inputs_true() {
   %result = call i64 @llvm.amdgcn.fcmp.f32(float 2.0, float 4.0, i32 4)
   ret i64 %result
Index: lib/Transforms/InstCombine/InstCombineCalls.cpp
===================================================================
--- lib/Transforms/InstCombine/InstCombineCalls.cpp
+++ lib/Transforms/InstCombine/InstCombineCalls.cpp
@@ -3400,8 +3400,22 @@
     if (auto *CSrc0 = dyn_cast<Constant>(Src0)) {
       if (auto *CSrc1 = dyn_cast<Constant>(Src1)) {
         Constant *CCmp = ConstantExpr::getCompare(CCVal, CSrc0, CSrc1);
-        return replaceInstUsesWith(*II,
-                                   ConstantExpr::getSExt(CCmp, II->getType()));
+        if (CCmp->isNullValue()) {
+          return replaceInstUsesWith(*II,
+                                     ConstantExpr::getSExt(CCmp, II->getType()));
+        } else {
+          Value *NewF = Intrinsic::getDeclaration(II->getModule(),
+                                                  Intrinsic::read_register,
+                                                  II->getType());
+          Metadata *MDArgs[] = { MDString::get(II->getContext(), "exec") };
+          MDNode *MD = MDNode::get(II->getContext(), MDArgs);
+          Value *Args[] = { MetadataAsValue::get(II->getContext(), MD) };
+          CallInst *NewCall = Builder->CreateCall(NewF, Args);
+          NewCall->addAttribute(AttributeList::FunctionIndex,
+                                Attribute::Convergent);
+          NewCall->takeName(II);
+          return replaceInstUsesWith(*II, NewCall);
+        }
       }
 
       // Canonicalize constants to RHS.


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