basic asm and memory clobbers

[Andrew Haley]

On 17/11/15 05:55, David Wohlferd wrote:
> How much pessimism are we talking here?  Wouldn't clobbering everything 
> effectively force the reloading of (some? most? all?) registers?  And 
> more memory will be needed to store things that used to just require 
> registers?

No, really not.  It only affects locals whose address has been taken
and is live at that point.

> Along with a few more memory writes?  A single line of basic asm,
> even a comment, could have a non-trivial impact on the code that
> gets generated.
> 
> One common use I've seen for basic asm is "int $3" on x86 to break into 
> the debugger (the basic asm docs use this as a sample). Changing this to 
> a clobber-everything will make what used to be a minimally intrusive way 
> to debug code into a high impact operation that may obscure the very 
> thing being debugged.

I think that's really pretty unlikely.  Besides, if you need a
breakpoint for debugging you really don't want operations to move
across that breakpoint.

Andrew.