Re: Using cc (question from avr)
Paulo J. Matos schrieb: Hi, I am having some trouble really understanding the working of cc_status. In order to understand it better I was looking at the code for avr under gcc 4.3. My assumption is that set_zn, set_* means that an instructions _changes_ these flags. So an instruction that set_zn means that Z and N are modified but we are not sure to which values. I assume clobber means that an instruction changes some flags but we don't know which. set_* tells you which condition code flags have been set in a usable way, e.g. to be used in a subsequent branch instruction. For example, if * contains 'z' you can use the Z-Flag to test if the value just set is equal to zero or not. Look at andqi3 insn which sets Z and N, but andhi3 just sets N or clobbers CC. Z just reflects the andi-action of the high byte of the result, so you cannot use Z-flag to test wether or not the result is zero or not. However, for alternatives 0 and 2, you can use N to test the result (interpreted as signed) for 0 resp. = 0. 'clobber' means the instruction leaves CC in a mess. 'none' means CC is unchanged. Now, the first thing that surprises me is clobber. Given a processor instruction description we know exactly which instructions set what. What's the need for clobber? Check the following example from avr: (define_insn *strlenhi [(set (match_operand:HI 0 register_operand =e) (unspec:HI [(mem:BLK (match_operand:HI 1 register_operand %0)) (const_int 0) (match_operand:HI 2 immediate_operand i)] UNSPEC_STRLEN))] ld __tmp_reg__,%a0+ tst __tmp_reg__ brne .-6 [(set_attr length 3) (set_attr cc clobber)]) From the instruction manual I have [1] ld changes none, tst changes Z,N and brne changes none so I would expect this instruction to have (set_attr cc set_zn) instead of the clobber. Why is this? No. Z will always be set after that insn, even if op0 is not zero. Thus, Z has nothing to do with op0, dito for N. Therefore, CC is 'clobber' because it is not unchanged. Johann
Re: Using cc (question from avr)
On Mar 9, 2011, at 3:35 PM, Georg-Johann Lay wrote: Paulo J. Matos schrieb: Hi, I am having some trouble really understanding the working of cc_status. In order to understand it better I was looking at the code for avr under gcc 4.3. My assumption is that set_zn, set_* means that an instructions _changes_ these flags. So an instruction that set_zn means that Z and N are modified but we are not sure to which values. I assume clobber means that an instruction changes some flags but we don't know which. set_* tells you which condition code flags have been set in a usable way, e.g. to be used in a subsequent branch instruction. For example, if * contains 'z' you can use the Z-Flag to test if the value just set is equal to zero or not. Look at andqi3 insn which sets Z and N, but andhi3 just sets N or clobbers CC. Z just reflects the andi-action of the high byte of the result, so you cannot use Z-flag to test wether or not the result is zero or not. However, for alternatives 0 and 2, you can use N to test the result (interpreted as signed) for 0 resp. = 0. 'clobber' means the instruction leaves CC in a mess. 'none' means CC is unchanged. By the way: in a mess doesn't necessarily mean in an unpredicable way. It can also mean in a way that is too hard to describe, or not worth describing. For example, the strlen pattern quoted by Paulo would leave the condition codes set from the zero byte (i.e., Z=1). That could be stated, but it isn't interesting, so clobber simply tells GCC that nothing useful is in the CC at this time. That's true for the use of the term clobber generally, not just for condition codes. paul
