Re: A question about integer promotion in GCC

[Jie Zhang <zhangjie@magima.com.cn>]

Joseph S. Myers wrote:
> On Thu, 2 Sep 2004, Jie Zhang wrote:
>>According to this, shouldn't it be:
>>
>>    return (int)x << 8 | (int)x >> 8;
>>
> 
> The current function of tree dumps is for debugging the compiler, not as a 
> representation of source.  Various optimisations are performed on the 
> trees generated, both in the process of generating them to avoid 
> generating unnecessary garbage, and as part of fold(), before they get to 
> the first tree dumps.  (In this case, a right shift of a short can be 
> represented directly on the short, whereas a left shift of a short 
> cannot.)
> 

I think optimizing "(int)x >> 8" to "(int)(x >> 8)" is not good, at least for RISCs, like MIPS. Now gcc (3.4.0) generates the following assembly with option -O0:

  lhu     $2,0($fp)
  sll     $2,$2,8
  lhu     $3,0($fp)
  srl     $3,$3,8
  andi    $3,$3,0xffff
  or      $2,$2,$3

IMO, the "andi" instruction is useless. I suspect it's due to this optimization. Without knowing it this assembly is confusing.

> There is a mood towards doing less such optimisations at parse time (and 

I like this idea. It will make both the structure of GCC and the -O0 assembly output more clear.


regards
-- 
Jie