Add C API for thread yielding callback.

Sometimes a LLVM compilation may take more time then a client would like to
wait for. The problem is that it is not possible to safely suspend the LLVM
thread from the outside. When the timing is bad it might be possible that the
LLVM thread holds a global mutex and this would block any progress in any other
thread.

This commit adds a new yield callback function that can be registered with a
context. LLVM will try to yield by calling this callback function, but there is
no guaranteed frequency. LLVM will only do so if it can guarantee that
suspending the thread won't block any forward progress in other LLVM contexts
in the same process.

Once the client receives the call back it can suspend the thread safely and
resume it at another time.

Related to <rdar://problem/16728690>

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@208945 91177308-0d34-0410-b5e6-96231b3b80d8

1 files changed, 27 insertions, 0 deletions

diff --git a/unittests/ExecutionEngine/MCJIT/MCJITCAPITest.cpp b/unittests/ExecutionEngine/MCJIT/MCJITCAPITest.cpp
index 3813d59dbd..20d3f139b4 100644
--- a/unittests/ExecutionEngine/MCJIT/MCJITCAPITest.cpp
+++ b/unittests/ExecutionEngine/MCJIT/MCJITCAPITest.cpp
@@ -28,6 +28,7 @@ using namespace llvm;
 
 static bool didCallAllocateCodeSection;
 static bool didAllocateCompactUnwindSection;
+static bool didCallYield;
 
 static uint8_t *roundTripAllocateCodeSection(void *object, uintptr_t size,
                                              unsigned alignment,
@@ -64,6 +65,10 @@ static void roundTripDestroy(void *object) {
   delete static_cast<SectionMemoryManager*>(object);
 }
 
+static void yield(LLVMContextRef, void *) {
+  didCallYield = true;
+}
+
 namespace {
 
 // memory manager to test reserve allocation space callback
@@ -142,6 +147,7 @@ protected:
   virtual void SetUp() {
     didCallAllocateCodeSection = false;
     didAllocateCompactUnwindSection = false;
+    didCallYield = false;
     Module = 0;
     Function = 0;
     Engine = 0;
@@ -429,3 +435,24 @@ TEST_F(MCJITCAPITest, reserve_allocation_space) {
   EXPECT_TRUE(MM->UsedCodeSize > 0); 
   EXPECT_TRUE(MM->UsedDataSizeRW > 0);
 }
+
+TEST_F(MCJITCAPITest, yield) {
+  SKIP_UNSUPPORTED_PLATFORM;
+
+  buildSimpleFunction();
+  buildMCJITOptions();
+  buildMCJITEngine();
+  LLVMContextRef C = LLVMGetGlobalContext();
+  LLVMContextSetYieldCallback(C, yield, NULL);
+  buildAndRunPasses();
+
+  union {
+    void *raw;
+    int (*usable)();
+  } functionPointer;
+  functionPointer.raw = LLVMGetPointerToGlobal(Engine, Function);
+
+  EXPECT_EQ(42, functionPointer.usable());
+  EXPECT_TRUE(didCallYield);
+}
+