[AMDGPU] Unroll preferences improvements

Exit loop analysis early if suitable private access found.
Do not account for GEPs which are invariant to loop induction variable.
Do not account for Allocas which are too big to fit into register file anyway.
Add option for tuning: -amdgpu-unroll-threshold-private.

Differential Revision: https://reviews.llvm.org/D29473

llvm-svn: 293991

llvm/lib/Target/AMDGPU/AMDGPUTargetTransformInfo.cpp

@@ -29,6 +29,10 @@ using namespace llvm;
 
 #define DEBUG_TYPE "AMDGPUtti"
 
+static cl::opt<unsigned> UnrollThresholdPrivate(
+  "amdgpu-unroll-threshold-private",
+  cl::desc("Unroll threshold for AMDGPU if private memory used in a loop"),
+  cl::init(800), cl::Hidden);
 
 void AMDGPUTTIImpl::getUnrollingPreferences(Loop *L,
                                             TTI::UnrollingPreferences &UP) {
@@ -38,6 +42,8 @@ void AMDGPUTTIImpl::getUnrollingPreferences(Loop *L,
 
   // TODO: Do we want runtime unrolling?
 
+  // Maximum alloca size than can fit registers. Reserve 16 registers.
+  const unsigned MaxAlloca = (256 - 16) * 4;
   for (const BasicBlock *BB : L->getBlocks()) {
     const DataLayout &DL = BB->getModule()->getDataLayout();
     for (const Instruction &I : *BB) {
@@ -49,6 +55,26 @@ void AMDGPUTTIImpl::getUnrollingPreferences(Loop *L,
       const AllocaInst *Alloca =
           dyn_cast<AllocaInst>(GetUnderlyingObject(Ptr, DL));
       if (Alloca) {
+        Type *Ty = Alloca->getAllocatedType();
+        unsigned AllocaSize = Ty->isSized() ? DL.getTypeAllocSize(Ty) : 0;
+        if (AllocaSize > MaxAlloca)
+          continue;
+
+        // Check if GEP depends on a value defined by this loop itself.
+        bool HasLoopDef = false;
+        for (const Value *Op : GEP->operands()) {
+          const Instruction *Inst = dyn_cast<Instruction>(Op);
+          if (!Inst || L->isLoopInvariant(Op))
+            continue;
+          if (any_of(L->getSubLoops(), [Inst](const Loop* SubLoop) {
+               return SubLoop->contains(Inst); }))
+            continue;
+          HasLoopDef = true;
+          break;
+        }
+        if (!HasLoopDef)
+          continue;
+
         // We want to do whatever we can to limit the number of alloca
         // instructions that make it through to the code generator.  allocas
         // require us to use indirect addressing, which is slow and prone to
@@ -59,7 +85,8 @@ void AMDGPUTTIImpl::getUnrollingPreferences(Loop *L,
         //
         // Don't use the maximum allowed value here as it will make some
         // programs way too big.
-        UP.Threshold = 800;
+        UP.Threshold = UnrollThresholdPrivate;
+        return;
       }
     }
   }

llvm/test/Transforms/LoopUnroll/AMDGPU/unroll-for-private.ll

@@ -0,0 +1,120 @@
+; RUN: opt -mtriple=amdgcn-unknown-amdhsa -loop-unroll -S -amdgpu-unroll-threshold-private=20000 %s | FileCheck %s
+
+; Check that we full unroll loop to be able to eliminate alloca
+; CHECK-LABEL: @non_invariant_ind
+; CHECK:       for.body:
+; CHECK-NOT:   br
+; CHECK:       store i32 %tmp15, i32 addrspace(1)* %arrayidx7, align 4
+; CHECK:       ret void
+
+define void @non_invariant_ind(i32 addrspace(1)* nocapture %a, i32 %x) {
+entry:
+  %arr = alloca [64 x i32], align 4
+  %tmp1 = tail call i32 @llvm.amdgcn.workitem.id.x() #1
+  br label %for.body
+
+for.cond.cleanup:                                 ; preds = %for.body
+  %arrayidx5 = getelementptr inbounds [64 x i32], [64 x i32]* %arr, i32 0, i32 %x
+  %tmp15 = load i32, i32* %arrayidx5, align 4
+  %arrayidx7 = getelementptr inbounds i32, i32 addrspace(1)* %a, i32 %tmp1
+  store i32 %tmp15, i32 addrspace(1)* %arrayidx7, align 4
+  ret void
+
+for.body:                                         ; preds = %for.body, %entry
+  %i.015 = phi i32 [ 0, %entry ], [ %inc, %for.body ]
+  %idxprom = sext i32 %i.015 to i64
+  %arrayidx = getelementptr inbounds i32, i32 addrspace(1)* %a, i64 %idxprom
+  %tmp16 = load i32, i32 addrspace(1)* %arrayidx, align 4
+  %add = add nsw i32 %i.015, %tmp1
+  %rem = srem i32 %add, 64
+  %arrayidx3 = getelementptr inbounds [64 x i32], [64 x i32]* %arr, i32 0, i32 %rem
+  store i32 %tmp16, i32* %arrayidx3, align 4
+  %inc = add nuw nsw i32 %i.015, 1
+  %exitcond = icmp eq i32 %inc, 100
+  br i1 %exitcond, label %for.cond.cleanup, label %for.body
+}
+
+; Check that we unroll inner loop but not outer
+; CHECK-LABEL: @invariant_ind
+; CHECK:       %[[exitcond:[^ ]+]] = icmp eq i32 %{{.*}}, 32
+; CHECK:       br i1 %[[exitcond]]
+; CHECK-NOT:   icmp eq i32 %{{.*}}, 100
+
+define void @invariant_ind(i32 addrspace(1)* nocapture %a, i32 %x) {
+entry:
+  %arr = alloca [64 x i32], align 4
+  %tmp1 = tail call i32 @llvm.amdgcn.workitem.id.x() #1
+  br label %for.cond2.preheader
+
+for.cond2.preheader:                              ; preds = %for.cond.cleanup5, %entry
+  %i.026 = phi i32 [ 0, %entry ], [ %inc10, %for.cond.cleanup5 ]
+  %idxprom = sext i32 %i.026 to i64
+  %arrayidx = getelementptr inbounds i32, i32 addrspace(1)* %a, i64 %idxprom
+  %tmp15 = load i32, i32 addrspace(1)* %arrayidx, align 4
+  br label %for.body6
+
+for.cond.cleanup:                                 ; preds = %for.cond.cleanup5
+  %arrayidx13 = getelementptr inbounds [64 x i32], [64 x i32]* %arr, i32 0, i32 %x
+  %tmp16 = load i32, i32* %arrayidx13, align 4
+  %arrayidx15 = getelementptr inbounds i32, i32 addrspace(1)* %a, i32 %tmp1
+  store i32 %tmp16, i32 addrspace(1)* %arrayidx15, align 4
+  ret void
+
+for.cond.cleanup5:                                ; preds = %for.body6
+  %inc10 = add nuw nsw i32 %i.026, 1
+  %exitcond27 = icmp eq i32 %inc10, 32
+  br i1 %exitcond27, label %for.cond.cleanup, label %for.cond2.preheader
+
+for.body6:                                        ; preds = %for.body6, %for.cond2.preheader
+  %j.025 = phi i32 [ 0, %for.cond2.preheader ], [ %inc, %for.body6 ]
+  %add = add nsw i32 %j.025, %tmp1
+  %rem = srem i32 %add, 64
+  %arrayidx8 = getelementptr inbounds [64 x i32], [64 x i32]* %arr, i32 0, i32 %rem
+  store i32 %tmp15, i32* %arrayidx8, align 4
+  %inc = add nuw nsw i32 %j.025, 1
+  %exitcond = icmp eq i32 %inc, 100
+  br i1 %exitcond, label %for.cond.cleanup5, label %for.body6
+}
+
+; Check we do not enforce unroll if alloca is too big
+; CHECK-LABEL: @too_big
+; CHECK:       for.body:
+; CHECK:       icmp eq i32 %{{.*}}, 100
+; CHECK:       br
+
+define void @too_big(i32 addrspace(1)* nocapture %a, i32 %x) {
+entry:
+  %arr = alloca [256 x i32], align 4
+  %tmp1 = tail call i32 @llvm.amdgcn.workitem.id.x() #1
+  br label %for.body
+
+for.cond.cleanup:                                 ; preds = %for.body
+  %arrayidx5 = getelementptr inbounds [256 x i32], [256 x i32]* %arr, i32 0, i32 %x
+  %tmp15 = load i32, i32* %arrayidx5, align 4
+  %arrayidx7 = getelementptr inbounds i32, i32 addrspace(1)* %a, i32 %tmp1
+  store i32 %tmp15, i32 addrspace(1)* %arrayidx7, align 4
+  ret void
+
+for.body:                                         ; preds = %for.body, %entry
+  %i.015 = phi i32 [ 0, %entry ], [ %inc, %for.body ]
+  %idxprom = sext i32 %i.015 to i64
+  %arrayidx = getelementptr inbounds i32, i32 addrspace(1)* %a, i64 %idxprom
+  %tmp16 = load i32, i32 addrspace(1)* %arrayidx, align 4
+  %add = add nsw i32 %i.015, %tmp1
+  %rem = srem i32 %add, 64
+  %arrayidx3 = getelementptr inbounds [256 x i32], [256 x i32]* %arr, i32 0, i32 %rem
+  store i32 %tmp16, i32* %arrayidx3, align 4
+  %inc = add nuw nsw i32 %i.015, 1
+  %exitcond = icmp eq i32 %inc, 100
+  br i1 %exitcond, label %for.cond.cleanup, label %for.body
+}
+
+declare i8 addrspace(2)* @llvm.amdgcn.dispatch.ptr() #1
+
+declare i32 @llvm.amdgcn.workitem.id.x() #1
+
+declare i32 @llvm.amdgcn.workgroup.id.x() #1
+
+declare i8 addrspace(2)* @llvm.amdgcn.implicitarg.ptr() #1
+
+attributes #1 = { nounwind readnone }