[NVPTX] Emit .pragma "nounroll" for loops marked with nounroll

Summary:
CUDA driver can unroll loops when jit-compiling PTX. To prevent CUDA
driver from unrolling a loop marked with llvm.loop.unroll.disable is not
unrolled by CUDA driver, we need to emit .pragma "nounroll" at the
header of that loop.

This patch also extracts getting unroll metadata from loop ID metadata
into a shared helper function.

Test Plan: test/CodeGen/NVPTX/nounroll.ll

Reviewers: eliben, meheff, jholewinski

Reviewed By: jholewinski

Subscribers: jholewinski, llvm-commits

Differential Revision: http://reviews.llvm.org/D7041

llvm-svn: 227703

1 files changed, 40 insertions, 0 deletions

diff --git a/llvm/lib/Target/NVPTX/NVPTXAsmPrinter.cpp b/llvm/lib/Target/NVPTX/NVPTXAsmPrinter.cpp
index 73a2246513a..92e5a58372f 100644
--- a/llvm/lib/Target/NVPTX/NVPTXAsmPrinter.cpp
+++ b/llvm/lib/Target/NVPTX/NVPTXAsmPrinter.cpp
@@ -27,6 +27,7 @@
 #include "llvm/Analysis/ConstantFolding.h"
 #include "llvm/CodeGen/Analysis.h"
 #include "llvm/CodeGen/MachineFrameInfo.h"
+#include "llvm/CodeGen/MachineLoopInfo.h"
 #include "llvm/CodeGen/MachineModuleInfo.h"
 #include "llvm/CodeGen/MachineRegisterInfo.h"
 #include "llvm/IR/DebugInfo.h"
@@ -45,6 +46,7 @@
 #include "llvm/Support/TargetRegistry.h"
 #include "llvm/Support/TimeValue.h"
 #include "llvm/Target/TargetLoweringObjectFile.h"
+#include "llvm/Transforms/Utils/UnrollLoop.h"
 #include <sstream>
 using namespace llvm;
 
@@ -418,6 +420,44 @@ void NVPTXAsmPrinter::printReturnValStr(const MachineFunction &MF,
   printReturnValStr(F, O);
 }
 
+// Return true if MBB is the header of a loop marked with
+// llvm.loop.unroll.disable.
+// TODO(jingyue): consider "#pragma unroll 1" which is equivalent to "#pragma
+// nounroll".
+bool NVPTXAsmPrinter::isLoopHeaderOfNoUnroll(
+    const MachineBasicBlock &MBB) const {
+  MachineLoopInfo &LI = getAnalysis<MachineLoopInfo>();
+  // TODO(jingyue): isLoopHeader() should take "const MachineBasicBlock *".
+  // We insert .pragma "nounroll" only to the loop header.
+  if (!LI.isLoopHeader(const_cast<MachineBasicBlock *>(&MBB)))
+    return false;
+
+  // llvm.loop.unroll.disable is marked on the back edges of a loop. Therefore,
+  // we iterate through each back edge of the loop with header MBB, and check
+  // whether its metadata contains llvm.loop.unroll.disable.
+  for (auto I = MBB.pred_begin(); I != MBB.pred_end(); ++I) {
+    const MachineBasicBlock *PMBB = *I;
+    if (LI.getLoopFor(PMBB) != LI.getLoopFor(&MBB)) {
+      // Edges from other loops to MBB are not back edges.
+      continue;
+    }
+    if (const BasicBlock *PBB = PMBB->getBasicBlock()) {
+      if (const MDNode *LoopID =
+              PBB->getTerminator()->getMetadata("llvm.loop")) {
+        if (GetUnrollMetadata(LoopID, "llvm.loop.unroll.disable"))
+          return true;
+      }
+    }
+  }
+  return false;
+}
+
+void NVPTXAsmPrinter::EmitBasicBlockStart(const MachineBasicBlock &MBB) const {
+  AsmPrinter::EmitBasicBlockStart(MBB);
+  if (isLoopHeaderOfNoUnroll(MBB))
+    OutStreamer.EmitRawText(StringRef("\t.pragma \"nounroll\";\n"));
+}
+
 void NVPTXAsmPrinter::EmitFunctionEntryLabel() {
   SmallString<128> Str;
   raw_svector_ostream O(Str);