On Thu, 2011-04-14 at 21:19 +0800, Carrot Wei wrote: > On Fri, Apr 8, 2011 at 6:51 PM, Ramana Radhakrishnan > <ramana.radhakrish...@linaro.org> wrote: > > On 08/04/11 10:57, Carrot Wei wrote: > >> > >> Hi > >> > >> This is the second part of the fixing for > >> > >> http://gcc.gnu.org/bugzilla/show_bug.cgi?id=47855 > >> > >> This patch contains the length computation for insn patterns > >> "*arm_movqi_insn" > >> and "*arm_addsi3". Since the alternatives and encodings are much more > >> complex, > >> the attribute length is computed in separate C functions. > > > Sorry, no. This is potentially a maintenance pain. It hardcodes alternatives > > from a pattern elsewhere in the C file. I don't like doing this unless we > > have to with the sync primitives or with push_multi. In this case I'm not > > convinced we need such functions in the .c file. > > > > Why can't we use the "enabled" attribute here with appropriate constraints > > for everything other than the memory cases (even there we might be able to > > invent some new constraints) ? > > > > Also a note about programming style. There are the helper macros like REG_P, > > CONST_INT_P and MEM_P which remove the necessity for checks like > > > > GET_CODE (x) == y where y E { REG, CONST_INT, MEM} > > Hi Ramana > > As you suggested I created several new constraints, and use the > "enabled" attribute to split the current alternatives in this new > patch. It has been tested on arm qemu without regression. > > thanks > Carrot
Sorry, I don't think this approach can work. Certainly not with the way the compiler currently works, and especially for mov and add insns. These instructions are only 2 bytes long if either: 1) They clobber the condition code register or 2) They occur inside an IT block. We can't tell either of these from the pattern, so you're underestimating the length of the instruction in some circumstances by claiming that they are only 2 bytes long. That /will/ lead to broken code someday. We can't add potential clobbers to mov and add patterns because that will break reload which relies on these patterns being simple-set insns with no added baggage. It *might* be possible to add clobbers to other operations, but that will then most-likely upset instruction scheduling (I think the scheduler treats two insns that clobber the same hard reg as being strongly ordered). Putting in the clobber too early will certainly affect cond-exec generation. In short, I'm not aware of a simple way to address this problem so that we get accurate length information, but minimal impact on other passes in the compiler. R.
