Add __builtin_readcyclecounter() to produce the @llvm.readcyclecounter() intrinsic.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@161310 91177308-0d34-0410-b5e6-96231b3b80d8

docs/LanguageExtensions.html

@@ -98,6 +98,7 @@
 <li><a href="#complex-list-init">Initializer lists for complex numbers in C</a></li>
 <li><a href="#builtins">Builtin Functions</a>
   <ul>
+  <li><a href="#__builtin_readcyclecounter">__builtin_readcyclecounter</a></li>
   <li><a href="#__builtin_shufflevector">__builtin_shufflevector</a></li>
   <li><a href="#__builtin_unreachable">__builtin_unreachable</a></li>
   <li><a href="#__sync_swap">__sync_swap</a></li>
@@ -1366,6 +1367,42 @@ functions are implemented directly in terms of <a href="#vectors">extended
 vector support</a> instead of builtins, in order to reduce the number of
 builtins that we need to implement.</p>
 
+<!-- ======================================================================= -->
+<h3><a name="__builtin_readcyclecounter">__builtin_readcyclecounter</a></h3>
+<!-- ======================================================================= -->
+
+<p><tt>__builtin_readcyclecounter</tt> is used to access the cycle counter
+register (or a similar low-latency, high-accuracy clock) on those targets that
+support it.
+</p>
+
+<p><b>Syntax:</b></p>
+
+<pre>
+__builtin_readcyclecounter()
+</pre>
+
+<p><b>Example of Use:</b></p>
+
+<pre>
+unsigned long long t0 = __builtin_readcyclecounter();
+do_something();
+unsigned long long t1 = __builtin_readcyclecounter();
+unsigned long long cycles_to_do_something = t1 - t0; // assuming no overflow
+</pre>
+
+<p><b>Description:</b></p>
+
+<p>The __builtin_readcyclecounter() builtin returns the cycle counter value,
+which may be either global or process/thread-specific depending on the target.
+As the backing counters often overflow quickly (on the order of
+seconds) this should only be used for timing small intervals. When not
+supported by the target, the return value is always zero. This builtin
+takes no arguments and produces an unsigned long long result.
+</p>
+
+<p>Query for this feature with __has_builtin(__builtin_readcyclecounter).</p>
+
 <!-- ======================================================================= -->
 <h3><a name="__builtin_shufflevector">__builtin_shufflevector</a></h3>
 <!-- ======================================================================= -->

include/clang/Basic/Builtins.def

@@ -476,6 +476,7 @@ BUILTIN(__builtin___vprintf_chk, "iicC*a", "FP:1:")
 
 BUILTIN(__builtin_expect, "LiLiLi"   , "nc")
 BUILTIN(__builtin_prefetch, "vvC*.", "nc")
+BUILTIN(__builtin_readcyclecounter, "ULLi", "n")
 BUILTIN(__builtin_trap, "v", "nr")
 BUILTIN(__builtin_unreachable, "v", "nr")
 BUILTIN(__builtin_shufflevector, "v."   , "nc")

lib/CodeGen/CGBuiltin.cpp

@@ -365,6 +365,10 @@ RValue CodeGenFunction::EmitBuiltinExpr(const FunctionDecl *FD,
     Value *F = CGM.getIntrinsic(Intrinsic::prefetch);
     return RValue::get(Builder.CreateCall4(F, Address, RW, Locality, Data));
   }
+  case Builtin::BI__builtin_readcyclecounter: {
+    Value *F = CGM.getIntrinsic(Intrinsic::readcyclecounter);
+    return RValue::get(Builder.CreateCall(F));
+  }
   case Builtin::BI__builtin_trap: {
     Value *F = CGM.getIntrinsic(Intrinsic::trap);
     return RValue::get(Builder.CreateCall(F));

test/CodeGen/builtins.c

@@ -203,3 +203,9 @@ void test_builtin_longjmp(void **buffer) {
   __builtin_longjmp(buffer, 1);
   // CHECK-NEXT: unreachable
 }
+
+// CHECK: define i64 @test_builtin_readcyclecounter
+long long test_builtin_readcyclecounter() {
+  // CHECK: call i64 @llvm.readcyclecounter()
+  return __builtin_readcyclecounter();
+}