Force ldrd on armv7

Summary:
Clang generates suboptimal code for armv7 when loading a 64 bit value
with a register offset that is added rather than subtracted. For
example, when loading from a register, `frameRegs[-offset]` will
compile to:
```
sub.w   r0, r0, r1, lsl #3
ldrd    r0, r1, [r0]
bx      lr
```

Whereas `frameRegs[offset]` will compile to:
```
ldr.w   r2, [r0, r1, lsl #3]
add.w   r0, r0, r1, lsl #3
ldr     r1, [r0, #4]
mov     r0, r2
bx      lr
```

By using inline asm, we can force clang to perform the add as a
separate operation before the load, which prevents it from being folded
into the load and allows it to use the more efficient 64 bit load
instead.

Reviewed By: kodafb

Differential Revision: D31030620

fbshipit-source-id: 9620205e6382943c4a113d00c42d558bb5fbcdfc

CMakeLists.txt

@@ -149,6 +149,9 @@ set(HERMESVM_HEAP_SEGMENT_SIZE_KB 4096
 set(HERMESVM_ALLOW_CONCURRENT_GC ON CACHE BOOL
         "Enable concurrency in the GC for 64-bit builds.")
 
+set(HERMESVM_ALLOW_INLINE_ASM ON CACHE BOOL
+        "Allow the use of inline assembly in VM code.")
+
 set(HERMESVM_API_TRACE_ANDROID_REPLAY OFF CACHE BOOL
   "Simulate Android config on Linux in API tracing.")
 
@@ -349,6 +352,9 @@ add_definitions(-DHERMESVM_HEAP_SEGMENT_SIZE_KB=${HERMESVM_HEAP_SEGMENT_SIZE_KB}
 if(HERMESVM_ALLOW_CONCURRENT_GC)
     add_definitions(-DHERMESVM_ALLOW_CONCURRENT_GC)
 endif()
+if(HERMESVM_ALLOW_INLINE_ASM)
+    add_definitions(-DHERMESVM_ALLOW_INLINE_ASM)
+endif()
 if(HERMESVM_API_TRACE_ANDROID_REPLAY)
     add_definitions(-DHERMESVM_API_TRACE_ANDROID_REPLAY)
 endif()

lib/VM/Interpreter-internal.h

@@ -11,7 +11,23 @@
 #include "hermes/VM/Interpreter.h"
 
 // Convenient aliases for operand registers.
+#if !defined(__arm__) || !defined(__clang__) || \
+    !defined(HERMESVM_ALLOW_INLINE_ASM)
 #define REG(index) frameRegs[index]
+#else
+// Work around bad codegen from clang on armv7 that causes it to emit two
+// separate loads instead of a single ldrd.
+#define REG(index)                                                           \
+  (*({                                                                       \
+    PinnedHermesValue *addr;                                                 \
+    static_assert(sizeof(PinnedHermesValue) == 8, "asm depends on HV size"); \
+    static_assert(sizeof(index) <= 4, "index must fit in a register");       \
+    asm("add.w   %0, %1, %2, lsl #3"                                         \
+        : "=r"(addr)                                                         \
+        : "r"(frameRegs), "r"(index));                                       \
+    addr;                                                                    \
+  }))
+#endif
 #define O1REG(name) REG(ip->i##name.op1)
 #define O2REG(name) REG(ip->i##name.op2)
 #define O3REG(name) REG(ip->i##name.op3)

lib/VM/Interpreter.cpp

@@ -719,7 +719,8 @@ static void printDebugInfo(
     if (operandType == OperandType::Reg8 || operandType == OperandType::Reg32) {
       // Print the register value, if source.
       if (i != 0 || decoded.meta.numOperands == 1)
-        dbgs() << "=" << DumpHermesValue(REG(value.integer));
+        dbgs() << "="
+               << DumpHermesValue(REG(static_cast<uint32_t>(value.integer)));
     }
   }