On Wed, 2015-11-11 at 09:56 -0500, Rich Felker wrote: > Sorry, I don't really understand RTL well enough to make a code > snippet. What I want to express is that an insn "uses" (in the (use > ...) sense) a register (r12) conditionally depending on a runtime > option (TARGET_FDPIC).
As far as I know this is not possible. It would require two variants of the same pattern, one with the use and another without the use. > Is this possible in the sh backend or does it need changes to > higher-level gcc code? (i.e. is it presently possible to make an insn > that conditionally clobbers different things rather than having to > make tons of different insns for each possible set of clobbers?) This is basically the same as above ... it's not possible to conditionally construct/modify pattern descriptions in the .md. However, it's possible to modify the CALL_INSN_FUNCTION_USAGE field of call insns -- for some examples see 'grep -r CALL_INSN_FUNCTION_USAGE gcc/config/*'. Also, it seems the SH backend doesn't make use of some existing libcall related parameters and target hooks/macros. Maybe those could be helpful. > Another issue I've started looking at is how r12 is put in > fixed_regs, which is conceptually wrong. Preliminary tests show that > removing it from fixed_regs doesn't break and produces much better > code -- r12 gets used as a temp register in functions that don't need > it, and in one function that made multiple calls, the saving of > initial r12 to a call-saved register even happened in the delay slot > of the call. I've been discussing it with Alexander Monakov on IRC > (#musl) and based on my understanding so far of how gcc works (which > admittedly may be wrong) the current FDPIC code looks like it's > written not to depend on r12 being 'fixed'. Also I think I'm pretty > close to understanding how we could make the same improvements for > non-FDPIC PIC codegen: instead loading r12 in the prologue, load a > pseudo, then use that pseudo for GOT access and force it into r12 the > same way FDPIC call code does for PLT calls. Does this sound correct? Maybe TARGET_USE_PSEUDO_PIC_REG could be useful? Cheers, Oleg
