[FRONTEND] faster jit function launch (#523)

With fast (200 ns) get_stream function soon to be available from pytorch this shaves off approx 25-30 us from function launch, but even without that function due to caching device properties we are saving ~15-20us.

python/triton/code_gen.py

@@ -23,12 +23,17 @@ import triton
 import triton._C.libtriton.triton as _triton
 from .tools.disasm import extract
 
+try:
+    from torch._C import _cuda_getCurrentRawStream as get_cuda_stream
+except ImportError:
+    get_cuda_stream = lambda dev_idx: torch.cuda.current_stream(dev_idx).cuda_stream
+
 
 def current_cuda_stream(device_idx=0):
     # Torch's torch.cuda.current_stream() is slow. We provide this
     # function to give the user an opportunity to monkey-patch their
     # own faster current stream lookup.
-    return torch.cuda.current_stream().cuda_stream
+    return get_cuda_stream(device_idx)
 
 
 def mangle_ty(ty):
@@ -910,6 +915,7 @@ class Kernel:
 
     def __init__(self, fn):
         self.fn = fn
+        self.cache_key = {}
 
     def add_to_cache(self, key, wargs, device_idx, num_warps, num_stages):
         tensor_idxs = [i for i, arg in enumerate(wargs) if hasattr(arg, 'data_ptr')]
@@ -951,12 +957,11 @@ class Kernel:
         #         assert arg.is_cuda, "All tensors must be on GPU!"
         # set device (i.e., make sure torch has the context initialized)
         device = torch.cuda.current_device()
-        torch.cuda.set_device(device)
-        # query compute capability
-        cc = torch.cuda.get_device_capability(device)
-        cc = str(cc[0]) + '-' + str(cc[1])
-        cache_key = self.fn.cache_key + cc
-        # query current stream
+        if device not in self.cache_key:
+            cc = torch.cuda.get_device_capability(device)
+            cc = str(cc[0]) + '-' + str(cc[1])
+            self.cache_key[device] = self.fn.cache_key + cc
+        cache_key = self.cache_key[device]
         stream = current_cuda_stream(device)
         return _triton.runtime.launch(wargs, self.fn.do_not_specialize, cache_key, self.fn.arg_names,
                                       device, stream, self.fn.bin_cache, num_warps, num_stages, self.add_to_cache,