Re: Can I force a word read-modify-write instread of a byte write?

[Andrew Haley <aph-gcc at littlepinkcloud dot COM>]

Ian Lance Taylor writes:
 > "Phil Endecott" <spam_from_gcc_help@chezphil.org> writes:
 > 
 > > Could you help me out with an example?  Say I have a uint8_t* that I
 > > want to write to:
 > > 
 > > uint8_t* ptr = ......;
 > > *ptr = 123;
 > > 
 > > How do I change that so that it does a read/modify/write?  Are you
 > > saying that I can do something like
 > > 
 > > volatile uint32_t* ptr = .....;
 > > uint8_t* ptr2 = ptr;
 > > *ptr2 = 123;
 > > 
 > > with appropriate extra casts and 'volatile's, and it will do what I want?
 > 
 > Oh, I see, you want the compiler to figure stuff out for you.
 > Unfortunately, that won't work.  You need to write the 32-bit
 > read/modify/write instructions yourself.
 > 
 > volatile uint32_t* ptr = .....;
 > uint32_t v = *ptr;
 > v = (v & 0xffffff00) | 123;
 > *ptr = v;
 > 
 > Note that if you don't use the volatile qualifier, gcc is wholly
 > capable of optimizing that into an 8-bit memory write.
 > 
 > There is nothing in gcc which will force it to use 32-bit memory
 > accesses if you don't explicitly write 32-bit memory accesses.

You could hack the md file to add a new predicate to 8-bit memory
accesses.  Wouldn't that cause gcc to generate the necessary 32-bit
shifts and masks?

Andrew.