"Radu Hobincu" <radu.hobi...@arh.pub.ro> writes: > I have written here a few weeks ago regarding some tutorials on GCC > porting and got some very interesting replies. However, I seem to have > gotten stuck with a couple of issues in spite of my massive Googling, and > I was wondering if anyone could spare a couple of minutes for some > clarifications. > > I am having troubles with the condition codes (cc_status). I have looked > over a couple of architectures and I do not seem to understand how they > work. > > The machine I am porting GCC for has 1 4bit status register for carry, > zero, less than and equal. I do not have explicit comparison instructions, > all of the ALU instructions modify one or more flags. > > What I figured out so far looking over AVR and Cris machine descriptions > is that each instruction that modifies the flags contain an attr > declaration which specify what flags it is changing. Also, there is a > macro called NOTICE_UPDATE_CC which sets up the cc_status accordingly by > reading this attr. This is the part of the code I do not understand. There > are certain functions for which I could not find any descriptions, like > "single_set" and macros like "SET_DEST" and "SET_SRC". Also, looking over > conditions.h, I see that the CC_STATUS structure contains 2 rtx fields: > "value1" and "value2", and also an int called "flags". What do they > represent? Is "flags" the contents of the machine's flag register?
For a new port I recommend that you avoid cc0, cc_status, and NOTICE_UPDATE_CC. Instead, model the condition codes as 1 or 4 pseudo-registers. In your define_insn statements, include SET expressions which show how the condition code is updated. This is how the i386 backend works; see uses of FLAGS_REG in i386.md. As far as things like single_set, SET_DEST, and SET_SRC, you have reached the limits of the internal documentation. You have to open the source code and look at the comments. Similarly, the description of the CC_STATUS fields may be found in the comments above the definition of CC_STATUS in conditions.h. Ian
