Duplicate constants in stack lowering

[Quuxplusone]

Bugzilla Link	PR37299
Status	NEW
Importance	P enhancement
Reported by	kripken (alonzakai@gmail.com)
Reported on	2018-04-30 20:01:11 -0700
Last modified on	2018-05-31 14:44:27 -0700
Version	trunk
Hardware	PC Linux
CC	jgravelle@google.com, llvm-bugs@lists.llvm.org, llvm@sunfishcode.online
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The Souper superoptimizer finds stuff like this in the wasm backend's output,

; start LHS (in $luaL_error)
%0:i32 = var
; (i32.sub(get_local $5)(i32.const 128))
%1 = sub %0, 128:i32
; (i32.add(get_local $3)(i32.const 128))
%2 = add %1, 128:i32
infer %2

which corresponds to wasm like this

 (func $luaL_error (; 41 ;) (type $3) (param $var$0 i32) (param $var$1 i32) (param $var$2 i32) (result i32)
  (local $var$3 i32)
  (local $var$4 i32)
  (i32.store
   (i32.const 1024)
   (tee_local $var$3
    (i32.sub
     (i32.load
      (i32.const 1024)
     )
     (i32.const 128)
    )
   )
  )
  [..]
  (i32.store
   (i32.const 1024)
   (i32.add
    (get_local $var$3)
    (i32.const 128)
   )
  )
  (get_local $var$0)
 )

So it subs 128 from the stack pointer, then adds 128 to restore it. An
alternative would be to use a local to save the original value instead, which
would save a few bytes (avoid the add + constant at the end), although I'm not
sure how that would affect the size of the uses of the stack pointer in the
middle there.

[Quuxplusone]

We could do that, but it would be a tradeoff of register pressure for code size. In general that's probably not worth doing?

It shouldn't have an effect on the uses of the stack pointer for the rest of the function though.

We have logic to save stack frame and base pointers into locals when we need them, which is in the presence of VLAs and over-aligned allocations. If we do emit a frame pointer, our epilog just resets the stack pointer to the value of the frame pointer. In theory we could add "optimize for code size" as an input into making that decision, though I'm not sure if that's plumbed through to the backend.