[PowerPC] Loop Data Prefetching for the BG/Q

The IBM BG/Q supercomputer's A2 cores have a hardware prefetching unit, the
L1P, but it does not prefetch directly into the A2's L1 cache. Instead, it
prefetches into its own L1P buffer, and the latency to access that buffer is
significantly higher than that to the L1 cache (although smaller than the
latency to the L2 cache). As a result, especially when multiple hardware
threads are not actively busy, explicitly prefetching data into the L1 cache is
advantageous.

I've been using this pass out-of-tree for data prefetching on the BG/Q for well
over a year, and it has worked quite well. It is enabled by default only for
the BG/Q, but can be enabled for other cores as well via a command-line option.

Eventually, we might want to add some TTI interfaces and move this into
Transforms/Scalar (there is nothing particularly target dependent about it,
although only machines like the BG/Q will benefit from its simplistic
strategy).

llvm-svn: 229966

1 files changed, 15 insertions, 0 deletions

diff --git a/llvm/lib/Target/PowerPC/PPCTargetMachine.cpp b/llvm/lib/Target/PowerPC/PPCTargetMachine.cpp
index a81f7fbfc1c..b219e93dabe 100644
--- a/llvm/lib/Target/PowerPC/PPCTargetMachine.cpp
+++ b/llvm/lib/Target/PowerPC/PPCTargetMachine.cpp
@@ -43,6 +43,11 @@ EnableGEPOpt("ppc-gep-opt", cl::Hidden,
              cl::desc("Enable optimizations on complex GEPs"),
              cl::init(true));
 
+static cl::opt<bool>
+EnablePrefetch("enable-ppc-prefetching",
+                  cl::desc("disable software prefetching on PPC"),
+                  cl::init(false), cl::Hidden);
+
 extern "C" void LLVMInitializePowerPCTarget() {
   // Register the targets
   RegisterTargetMachine<PPC32TargetMachine> A(ThePPC32Target);
@@ -240,6 +245,16 @@ TargetPassConfig *PPCTargetMachine::createPassConfig(PassManagerBase &PM) {
 void PPCPassConfig::addIRPasses() {
   addPass(createAtomicExpandPass(&getPPCTargetMachine()));
 
+  // For the BG/Q (or if explicitly requested), add explicit data prefetch
+  // intrinsics.
+  bool UsePrefetching =
+    Triple(TM->getTargetTriple()).getVendor() == Triple::BGQ &&           
+    getOptLevel() != CodeGenOpt::None;
+  if (EnablePrefetch.getNumOccurrences() > 0)
+    UsePrefetching = EnablePrefetch;
+  if (UsePrefetching)
+    addPass(createPPCLoopDataPrefetchPass());
+
   if (TM->getOptLevel() == CodeGenOpt::Aggressive && EnableGEPOpt) {
     // Call SeparateConstOffsetFromGEP pass to extract constants within indices
     // and lower a GEP with multiple indices to either arithmetic operations or