Eliminate stack adjustment for float-returning functions

src/IceInstX86Base.h

Index: src/IceInstX86Base.h
diff --git a/src/IceInstX86Base.h b/src/IceInstX86Base.h
index c19ec9d63e2c651a37834fa965fb9b4a34e8433a..9a2c1113df8a2232d0d4ff3fa320a4963bafd670 100644
--- a/src/IceInstX86Base.h
+++ b/src/IceInstX86Base.h
@@ -405,8 +405,9 @@ private:
   InstX86Jmp(Cfg *Func, Operand *Target);
 };
 
-/// AdjustStack instruction - subtracts esp by the given amount and updates the
-/// stack offset during code emission.
+/// AdjustStack instruction - grows the stack (moves esp down) by the given

[Jim Stichnoth, 2015/11/13 23:30:33]

Optional.  I like your idea of breaking this into two pseudo-instructions, e.g.
GrowStack and ContractStack, which abstracts away the built-in assumption of
which direction the stack grows.

You could probably do this through minor template wizardry:

  template <class Machine>
  template <bool Grow>
  class InstX86AdjustStack ... {
    const uint32_t Amount;
    constexpr bool IsGrow = Grow;
  };

  ...
  using GrowStack = InstX86AdjustStack<Machine><true>;
  using ContractStack = InstX86AdjustStack<Machine><false>;

(Completely untested, but hopefully illustrative.)

[sehr, 2015/11/14 00:30:56]

I like the idea too (obviously), but can't make the template stuff work at the
moment.  I prefer to punt this from this CL.

+/// amount.  If the amount is negative, it shrinks the stack (moves esp up).
+/// It also updates the target lowering StackAdjustment during code emission.
 template <class Machine>
 class InstX86AdjustStack final : public InstX86Base<Machine> {
   InstX86AdjustStack() = delete;
@@ -414,7 +415,7 @@ class InstX86AdjustStack final : public InstX86Base<Machine> {
   InstX86AdjustStack &operator=(const InstX86AdjustStack &) = delete;
 
 public:
-  static InstX86AdjustStack *create(Cfg *Func, SizeT Amount, Variable *Esp) {
+  static InstX86AdjustStack *create(Cfg *Func, int32_t Amount, Variable *Esp) {
     return new (Func->allocate<InstX86AdjustStack>())
         InstX86AdjustStack(Func, Amount, Esp);
   }
@@ -427,8 +428,8 @@ public:
   }
 
 private:
-  InstX86AdjustStack(Cfg *Func, SizeT Amount, Variable *Esp);
-  SizeT Amount;
+  InstX86AdjustStack(Cfg *Func, int32_t Amount, Variable *Esp);
+  int32_t Amount;

[Jim Stichnoth, 2015/11/13 23:30:33]

Oops, this should be const.

[sehr, 2015/11/14 00:30:56]

Done.

 };
 
 /// Call instruction. Arguments should have already been pushed.