bindjcc optimization should take into account block coldness

Summary:
Suppose you have the following vasm:

`
  B1:
    .....
    jcc B2 B3
  B2:
    bindjmp
  B3 (cold):
    .....
`

the vasm-exits pass will convert this into:

`
  B1:
    .....
    bindjcc
    jmp B3
  B3 (cold):
    .....
`

then, since B1 ends in an unconditional jump to B3, the vasm-jumps pass will
merge B3 into B1. The problem is, B3 is cold, and B1 is not, so B3 won't be
emitted in the cold section.

Avoid this by only performing the above bindjcc optimization if the other
(non-bindjmp target) is not colder than the current block. With this logic, with
the above example, we'll emit a conditional jump into the cold section, and B1
will fallthru into the bindjmp (the common case).

I noticed this while making the reification errors in the function prologue be
marked as unlikely (which causes the above pattern).

Reviewed By: paulbiss

Differential Revision: D13020533

fbshipit-source-id: 0f7a2e15931084a251b36e872b354c49b8ef5405

hphp/runtime/vm/jit/irgen-func-prologue.cpp

@@ -128,6 +128,7 @@ void emitARHasReifiedGenericsCheck(IRGS& env) {
     },
     [&] {
       if (func->hasReifiedGenerics()) return;
+      hint(env, Block::Hint::Unlikely);
       gen(
         env,
         RaiseError,
@@ -137,6 +138,7 @@ void emitARHasReifiedGenericsCheck(IRGS& env) {
     [&] {
       if (!func->hasReifiedGenerics()) return;
       // null out VarEnv before raising error
+      hint(env, Block::Hint::Unlikely);
       gen(env, RaiseError, cns(env, s_reified_generics_not_given.get()));
     }
   );

hphp/runtime/vm/jit/vasm-exits.cpp

@@ -123,12 +123,17 @@ void optimizeExits(Vunit& unit) {
       code.emplace_back(jmp{next}, irctx);
     };
 
+    auto const is_colder = [&] (Vlabel succ) {
+      return unit.blocks[b].area_idx < unit.blocks[succ].area_idx;
+    };
+
     // Try to replace jcc to normal exit with bindexit followed by jmp,
     // as long as the sp adjustment is harmless to hoist (disp==0)
     Vptr sp;
-    if (match_bindjmp(unit, t1, &sp) && sp == sp.base[0]) {
+    if (match_bindjmp(unit, t1, &sp) && sp == sp.base[0] && !is_colder(t0)) {
       fold_exit(ijcc.cc, t1, t0);
-    } else if (match_bindjmp(unit, t0, &sp) && sp == sp.base[0]) {
+    } else if (match_bindjmp(unit, t0, &sp) && sp == sp.base[0] &&
+               !is_colder(t1)) {
       fold_exit(ccNegate(ijcc.cc), t0, t1);
     }
   });