(I've logged this as a potential speed improvement at LP: #681138 so we don't lose it)
-- Michael On Thu, Nov 25, 2010 at 8:29 AM, Peter Maydell <peter.mayd...@linaro.org> wrote: > This wiki page came up during the toolchain call: > https://wiki.linaro.org/Internal/People/KenWerner/AtomicMemoryOperations/ > > It gives the code generated for __sync_val_compare_and_swap > as including a push {r4} / pop {r4} pair because it uses too many > temporaries to fit them all in callee-saves registers. I think you > can tweak it a bit to get rid of that: > > # int __sync_val_compare_and_swap (int *mem, int old, int new); > # if the current value of *mem is old, then write new into *mem > # r0: mem, r1 old, r2 new > mov r3, r0 # move r0 into r3 > dmb sy # full memory barrier > .LSYT7: > ldrex r0, [r3] # load (exclusive) from memory pointed to > by r3 into r0 > cmp r0, r1 # compare contents of r0 (mem) with r1 > (old) -> updates the condition flag > bne .LSYB7 # branch to LSYB7 if mem != old > # This strex trashes the r0 we just loaded, but since we didn't take > # the branch we know that r0 == r1 > strex r0, r2, [r3] # store r2 (new) into memory pointed to > by r3 (mem) > # r0 contains 0 if the store was > successful, otherwise 1 > teq r0, #0 # compares contents of r0 with zero -> > updates the condition flag > bne .LSYT7 # branch to LSYT7 if r0 != 0 (if the > store wasn't successful) > # Move the value that was in memory into the right register to return > it > mov r0, r1 > dmb sy # full memory barrier > .LSYB7: > bx lr # return > > I think you can do a similar trick with __sync_fetch_and_add > (although you have to use a subtract to regenerate r0 from > r1 and r2). > > On the other hand I just looked at the gcc code that does this > and it's not simply dumping canned sequences out to the > assembler, so maybe it's not worth the effort just to drop a > stack push/pop. > > -- PMM > > _______________________________________________ > linaro-toolchain mailing list > linaro-toolchain@lists.linaro.org > http://lists.linaro.org/mailman/listinfo/linaro-toolchain > _______________________________________________ linaro-toolchain mailing list linaro-toolchain@lists.linaro.org http://lists.linaro.org/mailman/listinfo/linaro-toolchain
