Ian Lance Taylor writes: > "Phil Endecott" <spam_from_gcc_help@xxxxxxxxxxxx> 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.
