On Fri, Sep 21, 2012 at 9:01 AM, Tomasz Jankowski <tomcioj@xxxxxxxxx> wrote: > > For my engineering degree project I'm extending GCC backend for custom > architecture based on OpenRISC. First of all I have to understand how GCC > backend works (RTL via machine description to asm transformation). Which > machine description is the simplest and the best documented? I'm looking for > something to start from. OpenRISC machine description is sometimes "nasty". There is no great answer. All real machines have some oddities. Maybe you should look at moxie, since it exists primarily for a demonstration of porting to GCC, it might be cleaner than others. See http://moxielogic.org/blog . > I have to rewrite description for multiplication instruction. Now it takes > three 32 bit registers. Modified "mul" descriptions takes three 64 bit > numbers, when processors is still 32 bit. Each number is stored in two > continuous registers, where second register is used implicitly. Example: we > write "mul r3, r9, r11" and actually used registers will be: r3, r4, r9, > r10, r11, r12. For "mul r2, r8, r20" used registers are r2, r3, r8, r9, r20, > r21. Can you give a hint how to do that? I assume, that I should somehow > preserve these implicit registers during one of RTL passes, but I don't know > which. I reviewed some API for RTL objects, but I don't know how to deal > with this issue. Use the appropriate machine mode and set HARD_REGNO_NREGS correctly. > Second task is SIMD add instruction. There's a lot of SIMD instructions in > x86 description, but it's too complicated at the beginning. Anything simple > to start with? I'm sorry, I don't really know how to answer that question. Basically you want to define a vector mode and implement add for that mode. > Going back to "define_expand", is there any way to access insn RTX object > for which this "define_expand" is called? I want to conditionally > insert additional > insn after current insn. define_expand creates an rtx, it does not access an rtx. If you want to conditionally insert an additional insn, just emit it before calling DONE. Ian
