[Polly] Add option to limit the maximal number of parallel threads.

[dpeixott]

LGTM with small test case demonstrating the command line option is respected.

[jdoerfert]

Updated, waiting for D4990 to be commited.

[jdoerfert]

Closed by commit rL219004 (authored by @jdoerfert).

lib/CodeGen/LoopGenerators.cpp

 
 #include "polly/ScopDetection.h"
 #include "polly/CodeGen/LoopGenerators.h"
 #include "llvm/Analysis/LoopInfo.h"
 #include "llvm/IR/DataLayout.h"
 #include "llvm/IR/Dominators.h"
 #include "llvm/IR/Module.h"
 #include "llvm/Transforms/Utils/BasicBlockUtils.h"
+#include "llvm/Support/CommandLine.h"
 
 using namespace llvm;
 using namespace polly;
 
+static cl::opt<int>
+    PollyNumThreads("polly-num-threads",
+                    cl::desc("Number of threads to use (0 = auto)"), cl::Hidden,
+                    cl::init(0));
+
 // We generate a loop of either of the following structures:
 //
 //              BeforeBB                      BeforeBB
 //                 |                             |
 //                 v                             v
 //              GuardBB                      PreHeaderBB
 //              /      |                         |   _____
 //     __  PreHeaderBB  |                        v  \/    |
Type *Params[] = {PointerType::getUnqual(FunctionType::get(
                           Builder.getVoidTy(), Builder.getInt8PtrTy(), false)),
                       Builder.getInt8PtrTy(), Builder.getInt32Ty(), IVType,
                       IVType, IVType};
 
     FunctionType *Ty = FunctionType::get(Builder.getVoidTy(), Params, false);
     F = Function::Create(Ty, Linkage, Name, M);
   }
 
-  Value *NumberOfThreads = Builder.getInt32(0);
+  Value *NumberOfThreads = Builder.getInt32(PollyNumThreads);
   Value *Args[] = {SubFn, SubFnParam, NumberOfThreads, LB, UB, Stride};
 
   Builder.CreateCall(F, Args);
 }
 
 Value *ParallelLoopGenerator::createCallGetWorkItem(Value *LBPtr,
                                                     Value *UBPtr) {
   const std::string Name = "GOMP_loop_runtime_next";