AMDGPU: Fix handling of infinite loops in fragment shaders

Summary:
Due to the fact that kill is just a normal intrinsic, even though it's
supposed to terminate the thread, we can end up with provably infinite
loops that are actually supposed to end successfully. The
AMDGPUUnifyDivergentExitNodes pass breaks up these loops, but because
there's no obvious place to make the loop branch to, it just makes it
return immediately, which skips the exports that are supposed to happen
at the end and hangs the GPU if all the threads end up being killed.

While it would be nice if the fact that kill terminates the thread were
modeled in the IR, I think that the structurizer as-is would make a mess if we
did that when the kill is inside control flow. For now, we just add a null
export at the end to make sure that it always exports something, which fixes
the immediate problem without penalizing the more common case. This means that
we sometimes do two "done" exports when only some of the threads enter the
discard loop, but from tests the hardware seems ok with that.

This fixes dEQP-VK.graphicsfuzz.while-inside-switch with radv.

Reviewers: arsenm, nhaehnle

Subscribers: kzhuravl, jvesely, wdng, yaxunl, dstuttard, tpr, t-tye, hiraditya, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D70781

llvm/lib/Target/AMDGPU/AMDGPUUnifyDivergentExitNodes.cpp

@@ -34,6 +34,7 @@
 #include "llvm/IR/InstrTypes.h"
 #include "llvm/IR/Instructions.h"
 #include "llvm/IR/Intrinsics.h"
+#include "llvm/IR/IRBuilder.h"
 #include "llvm/IR/Type.h"
 #include "llvm/InitializePasses.h"
 #include "llvm/Pass.h"
@@ -117,24 +118,58 @@ static bool isUniformlyReached(const LegacyDivergenceAnalysis &DA,
   return true;
 }
 
+static void removeDoneExport(Function &F) {
+  ConstantInt *BoolFalse = ConstantInt::getFalse(F.getContext());
+  for (BasicBlock &BB : F) {
+    for (Instruction &I : BB) {
+      if (IntrinsicInst *Intrin = llvm::dyn_cast<IntrinsicInst>(&I)) {
+        if (Intrin->getIntrinsicID() == Intrinsic::amdgcn_exp) {
+          Intrin->setArgOperand(6, BoolFalse); // done
+        } else if (Intrin->getIntrinsicID() == Intrinsic::amdgcn_exp_compr) {
+          Intrin->setArgOperand(4, BoolFalse); // done
+        }
+      }
+    }
+  }
+}
+
 static BasicBlock *unifyReturnBlockSet(Function &F,
                                        ArrayRef<BasicBlock *> ReturningBlocks,
+                                       bool InsertExport,
                                        const TargetTransformInfo &TTI,
                                        StringRef Name) {
   // Otherwise, we need to insert a new basic block into the function, add a PHI
   // nodes (if the function returns values), and convert all of the return
   // instructions into unconditional branches.
   BasicBlock *NewRetBlock = BasicBlock::Create(F.getContext(), Name, &F);
+  IRBuilder<> B(NewRetBlock);
+
+  if (InsertExport) {
+    // Ensure that there's only one "done" export in the shader by removing the
+    // "done" bit set on the original final export. More than one "done" export
+    // can lead to undefined behavior.
+    removeDoneExport(F);
+
+    Value *Undef = UndefValue::get(B.getFloatTy());
+    B.CreateIntrinsic(Intrinsic::amdgcn_exp, { B.getFloatTy() },
+                      {
+                        B.getInt32(9), // target, SQ_EXP_NULL
+                        B.getInt32(0), // enabled channels
+                        Undef, Undef, Undef, Undef, // values
+                        B.getTrue(), // done
+                        B.getTrue(), // valid mask
+                      });
+  }
 
   PHINode *PN = nullptr;
   if (F.getReturnType()->isVoidTy()) {
-    ReturnInst::Create(F.getContext(), nullptr, NewRetBlock);
+    B.CreateRetVoid();
   } else {
     // If the function doesn't return void... add a PHI node to the block...
-    PN = PHINode::Create(F.getReturnType(), ReturningBlocks.size(),
+    PN = B.CreatePHI(F.getReturnType(), ReturningBlocks.size(),
-                         "UnifiedRetVal");
+                     "UnifiedRetVal");
-    NewRetBlock->getInstList().push_back(PN);
+    assert(!InsertExport);
-    ReturnInst::Create(F.getContext(), PN, NewRetBlock);
+    B.CreateRet(PN);
   }
 
   // Loop over all of the blocks, replacing the return instruction with an
@@ -173,6 +208,8 @@ bool AMDGPUUnifyDivergentExitNodes::runOnFunction(Function &F) {
   // Dummy return block for infinite loop.
   BasicBlock *DummyReturnBB = nullptr;
 
+  bool InsertExport = false;
+
   for (BasicBlock *BB : PDT.getRoots()) {
     if (isa<ReturnInst>(BB->getTerminator())) {
       if (!isUniformlyReached(DA, *BB))
@@ -188,6 +225,36 @@ bool AMDGPUUnifyDivergentExitNodes::runOnFunction(Function &F) {
                                            "DummyReturnBlock", &F);
         Type *RetTy = F.getReturnType();
         Value *RetVal = RetTy->isVoidTy() ? nullptr : UndefValue::get(RetTy);
+
+        // For pixel shaders, the producer guarantees that an export is
+        // executed before each return instruction. However, if there is an
+        // infinite loop and we insert a return ourselves, we need to uphold
+        // that guarantee by inserting a null export. This can happen e.g. in
+        // an infinite loop with kill instructions, which is supposed to
+        // terminate. However, we don't need to do this if there is a non-void
+        // return value, since then there is an epilog afterwards which will
+        // still export.
+        //
+        // Note: In the case where only some threads enter the infinite loop,
+        // this can result in the null export happening redundantly after the
+        // original exports. However, The last "real" export happens after all
+        // the threads that didn't enter an infinite loop converged, which
+        // means that the only extra threads to execute the null export are
+        // threads that entered the infinite loop, and they only could've
+        // exited through being killed which sets their exec bit to 0.
+        // Therefore, unless there's an actual infinite loop, which can have
+        // invalid results, or there's a kill after the last export, which we
+        // assume the frontend won't do, this export will have the same exec
+        // mask as the last "real" export, and therefore the valid mask will be
+        // overwritten with the same value and will still be correct. Also,
+        // even though this forces an extra unnecessary export wait, we assume
+        // that this happens rare enough in practice to that we don't have to
+        // worry about performance.
+        if (F.getCallingConv() == CallingConv::AMDGPU_PS &&
+            RetTy->isVoidTy()) {
+          InsertExport = true;
+        }
+
         ReturnInst::Create(F.getContext(), RetVal, DummyReturnBB);
         ReturningBlocks.push_back(DummyReturnBB);
       }
@@ -260,6 +327,6 @@ bool AMDGPUUnifyDivergentExitNodes::runOnFunction(Function &F) {
   const TargetTransformInfo &TTI
     = getAnalysis<TargetTransformInfoWrapperPass>().getTTI(F);
 
-  unifyReturnBlockSet(F, ReturningBlocks, TTI, "UnifiedReturnBlock");
+  unifyReturnBlockSet(F, ReturningBlocks, InsertExport, TTI, "UnifiedReturnBlock");
   return true;
 }

llvm/test/CodeGen/AMDGPU/kill-infinite-loop.ll

@@ -0,0 +1,68 @@
+; RUN: llc -march=amdgcn -verify-machineinstrs < %s | FileCheck -enable-var-scope %s
+; Although it's modeled without any control flow in order to get better code
+; out of the structurizer, @llvm.amdgcn.kill actually ends the thread that calls
+; it with "true". In case it's called in a provably infinite loop, we still
+; need to successfully exit and export something, even if we can't know where
+; to jump to in the LLVM IR. Therefore we insert a null export ourselves in
+; this case right before the s_endpgm to avoid GPU hangs, which is what this
+; tests.
+
+; CHECK-LABEL: return_void
+; Make sure that we remove the done bit from the original export
+; CHECK: exp mrt0 v{{[0-9]+}}, v{{[0-9]+}}, v{{[0-9]+}}, v{{[0-9]+}} vm
+; CHECK: exp null off, off, off, off done vm
+; CHECK-NEXT: s_endpgm
+define amdgpu_ps void @return_void(float %0) #0 {
+main_body:
+  %cmp = fcmp olt float %0, 1.000000e+01
+  br i1 %cmp, label %end, label %loop
+
+loop:
+  call void @llvm.amdgcn.kill(i1 false) #3
+  br label %loop
+
+end:
+  call void @llvm.amdgcn.exp.f32(i32 0, i32 15, float 0., float 0., float 0., float 1., i1 true, i1 true) #3
+  ret void
+}
+
+; Check that we also remove the done bit from compressed exports correctly.
+; CHECK-LABEL: return_void_compr
+; CHECK: exp mrt0 v{{[0-9]+}}, off, v{{[0-9]+}}, off compr vm
+; CHECK: exp null off, off, off, off done vm
+; CHECK-NEXT: s_endpgm
+define amdgpu_ps void @return_void_compr(float %0) #0 {
+main_body:
+  %cmp = fcmp olt float %0, 1.000000e+01
+  br i1 %cmp, label %end, label %loop
+
+loop:
+  call void @llvm.amdgcn.kill(i1 false) #3
+  br label %loop
+
+end:
+  call void @llvm.amdgcn.exp.compr.v2i16(i32 0, i32 5, <2 x i16> < i16 0, i16 0 >, <2 x i16> < i16 0, i16 0 >, i1 true, i1 true) #3
+  ret void
+}
+
+; In case there's an epilog, we shouldn't have to do this.
+; CHECK-LABEL: return_nonvoid
+; CHECK-NOT: exp null off, off, off, off done vm
+define amdgpu_ps float @return_nonvoid(float %0) #0 {
+main_body:
+  %cmp = fcmp olt float %0, 1.000000e+01
+  br i1 %cmp, label %end, label %loop
+
+loop:
+  call void @llvm.amdgcn.kill(i1 false) #3
+  br label %loop
+
+end:
+  ret float 0.
+}
+
+declare void @llvm.amdgcn.kill(i1) #0
+declare void @llvm.amdgcn.exp.f32(i32 immarg, i32 immarg, float, float, float, float, i1 immarg, i1 immarg) #0
+declare void @llvm.amdgcn.exp.compr.v2i16(i32 immarg, i32 immarg, <2 x i16>, <2 x i16>, i1 immarg, i1 immarg) #0
+
+attributes #0 = { nounwind }