"Mike Stump" <[EMAIL PROTECTED]> ??????:[EMAIL PROTECTED] > On Apr 8, 2007, at 2:37 AM, Uros Bizjak wrote: >> My docs say that "INC/DEC does not change the carry flag". > > Personally, I'm having a hard time envisioning how the semantics of the > instruction are relevant at all. This is all about instructing tuning, > so, semantics cannot matter, otherwise, it would be wrong to make this a > tune choice.
Intel's optimization reference manual says that: 3.5.1.1 Use of the INC and DEC Instructions The INC and DEC instructions modify only a subset of the bits in the flag register. This creates a dependence on all previous writes of the flag register. This is especially problematic when these instructions are on the critical path because they are used to change an address for a load on which many other instructions depend. Assembly/Compiler Coding Rule 32. (M impact, H generality) INC and DEC instructions should be replaced with ADD or SUB instructions, because ADD and SUB overwrite all flags, whereas INC and DEC do not, therefore creating false dependencies on earlier instructions that set the flags. > >> But you have better resources that I, so if you think that C2D should be >> left out of X86_TUNE_USE_INCDEC, then the patch is pre- approved for >> mainline. > > I'm confused again, it isn't that it should be left out, it is that it > should be included. My patch adds inc/dec selection for C2D. I'd also > like it for generic on darwin, as that makes more sense for us. How does > the rest of the community feel about inc/dec selection for generic? -- Zuxy
