Add thread safe guard variable code.

1 files changed, 26 insertions, 13 deletions

diff --git a/src/libc++/builtins.C b/src/libc++/builtins.C
index ed373c8cc..70c9fcc35 100644
--- a/src/libc++/builtins.C
+++ b/src/libc++/builtins.C
@@ -2,25 +2,38 @@
 
 extern "C" int __cxa_guard_acquire(volatile uint64_t* gv)
 {
-    // 0 .. uninitialized
-    // 1 .. locked
-    // 2 .. unlocked, initialized
-    if (0 == *gv)
-    {
-	*gv = 1;
-	return 1;
-    }
-    else if (1 == *gv)
+    // 0 -> uninitialized
+    // 1 -> locked
+    // 2 -> unlocked and initialized
+
+    register volatile void* guard = gv;
+    register uint32_t c = 0;
+
+    asm volatile(
+	"__cxa_guard_acquire_begin:"
+	"lwarx %0,0,%1;"	// Load guard with reserve
+	"cmpi 0,%0,0;"		// Compare with 0
+	"bne+ __cxa_guard_acquire_finish;"	// != 0, goto "finished"
+	"li %0, 1;"		// Set to 1.
+	"stwcx. %0,0,%1;"	// Store against reserve
+	"bne- __cxa_guard_acquire_begin;"	// goto begin if failed store.
+	"li %0, 3;"		// Set to 3 -> success in lock
+	"__cxa_guard_acquire_finish:" 
+	    : "+r" (c) : "r" (guard): "memory","cc"
+    );
+    while (2 > c)
     {
-	while(1 == *gv);
+	asm volatile("lwz %0, 0(%1);" : "=r" (c) : "r" (guard));
     }
-    
-    return 0;
+    return (3 == c ? 1 : 0);  // 3 means success in lock, return 1 (obtained)
+			      // 2 means initialized, return 0
 }
 
 extern "C" void __cxa_guard_release(volatile uint64_t* gv)
 {
-    *gv = 2;
+    register volatile void* guard = gv;
+    register uint32_t c = 2;
+    asm volatile("stw %0, 0(%1)" :: "r"(c) , "r" (guard): "memory");
     return;
 }