On Wed, Dec 17, 2008 at 7:14 AM, Ian Lance Taylor <i...@google.com> wrote: > "Guo, Xuepeng" <xuepeng....@intel.com> writes: > >> Thanks for your comments. It's not exactly the entire basic >> block. It should be from the current RTL to the end of the current >> basic block. As you know GCC will optimize "addl %ebx, %eax" to >> "leal (%ebx, %eax), %eax" to avoid the flag register dependency >> through a splitter in i386.md at split2 stage. But for my target, if >> the destination register of ADD actually holds non-address operand, >> which means "%eax" won't be used in MEM of the following RTL, this >> optimization will incur two cycles penalty. So I need to check >> whether the destination of ADD holds an address operands to decide >> whether split ADD to LEA. > > I gather that you are using basic block as a proxy for the next use of > the register, as the next use could presumably be after a branch > statement. > > You could probably get many uses very simply by checking the > REG_POINTER flag. It's not 100% reliable, but it's normally set on a > register which is used as a pointer.
We tried it and it doesn't work well due to 1) REG_POINTER(x) is documented as nonzero if a register x holds a pointer and stored in the frame-related filed. That may be accurate now. 2) Our LEA optimization happened at the stage of split2 after the IRA stage. The register x is a hard register and can hold anything it can hold. What we concern here is whether what register x holds will be used as an address soon and if it so we will use LEA, otherwise we will use ADD. In another word we can't judge register x holds a pointer only by REG_POINTER because there is no function to update this information step by step. > Otherwise you could use the dataflow framework to find the uses of the > register, but that may be too expensive since you can't easily control > the whole pass. > > Otherwise you will indeed have to write your own walker. This is easy > to write using for_each_rtx. > We will try that. Thanks. -- H.J.
