[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

1 files changed, 28 insertions, 1 deletions

diff --git a/llvm/lib/Target/AMDGPU/AMDGPUTargetTransformInfo.cpp b/llvm/lib/Target/AMDGPU/AMDGPUTargetTransformInfo.cpp
index 9fb68a38415..e48c55457b0 100644
--- a/llvm/lib/Target/AMDGPU/AMDGPUTargetTransformInfo.cpp
+++ b/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;
       }
     }
   }