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Re: Optimization breaks inline asm code w/ptrs


On Sun, Aug 13, 2017 at 10:25:14PM +0930, Alan Modra wrote:
> On Sun, Aug 13, 2017 at 03:35:15AM -0700, David Wohlferd wrote:
> > Using "m"(*pStr) as an (unused) input parameter has no effect.
> 
> Use "m" (*(const void *)pStr) and ignore the warning, or use
> "m" (*(const struct {char a; char x[];} *) pStr).

or even better "m" (*(const char (*)[]) pStr).

> The issue is one of letting gcc know what memory is accessed by the
> asm, if you don't want to use a "memory" clobber.  And there are very
> good reasons to avoid clobbering all memory.
> 
> "m"(*pStr) ought to work IMO, but apparently just tells gcc you are
> only interested in the first character.  Of course that is exactly
> what *pStr is, but in this context it would be nicer if it meant the
> entire array.

I take that back.  The relatively simple cast to differentiate a
pointer to a char from a pointer to an indeterminate length char array
makes it quite unnecessary for "m"(*pStr) to be treated as as array
reference.

I've opened https://gcc.gnu.org/bugzilla/show_bug.cgi?id=81845 to
track the lack of documentation.

-- 
Alan Modra
Australia Development Lab, IBM


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