On Thu, 2022-03-10 at 20:31 +0000, Qing Zhao wrote: > > > + SET_HARD_REG_BIT (zeroed_hardregs, HI_REGNUM); > > > + if (TEST_HARD_REG_BIT (need_zeroed_hardregs, LO_REGNUM)) > > > + SET_HARD_REG_BIT (zeroed_hardregs, LO_REGNUM); > > > + else > > > + emit_clobber (gen_rtx_REG (word_mode, LO_REGNUM)); > > > > …I don't think this conditional LO_REGNUM code is worth it. > > We might as well just add both registers to zeroed_hardregs. > > If the LO_REGNUM is NOT in “need_zeroed_hardregs”, adding it to > “zeroed_hardregs” seems not right to me. > What’s you mean by “not worth it”?
It's because the MIPS port almost always treat HI as "a subreg of dword HI-LO register". A direct "mthi $0" is possible but MIPS backend does not recognize "emit_move_insn (HI, CONST_0)". In theory it's possible to emit the mthi instruction explicitly here though, but we'll need to clear something NOT in need_zeroed_hardregs for MIPS anyway (see below). > > Here too I think we should just do: > > > > zeroed_hardregs |= reg_class_contents[ST_REGS] & accessible_reg_set; > > > > to include all available FCC registers. > > What’s the relationship between “ST_REGs” and FCC? (sorry for the stupid > question since I am not familiar with the MIPS register set). MIPS instruction manual names the 8 one-bit floating condition codes FCC0, ..., FCC7, but GCC MIPS backend code names the condition codes ST_REG0, ..., ST_REG7. Maybe it's better to always use the name "ST_REG" instead of "FCC" then. > From the above code, looks like that when any “ST_REGs” is in > “need_zeroed_hardregs”,FCC need to be cleared? Because there is no elegant way to clear one specific FCC bit in MIPS. A "ctc1 $0, $25" instruction will zero them altogether. If we really need to clear only one of them (let's say ST_REG3), we'll have to emit something like mtc1 $0, $0 # zero FPR0 to ensure it won't contain sNaN c.f.s $3, $0, $0 Then we'll still need to clobber FPR0 with zero. So anyway we'll have to clear some registers not specified in need_zeroed_hardregs. And the question is: is it really allowed to return something other than a subset of need_zeroed_hardregs for a TARGET_ZERO_CALL_USED_REGS hook? If yes then we'll happily to do so (like how the v2 of the patch does), otherwise we'd need to clobber those registers NOT in need_zeroed_hardregs explicitly. -- Xi Ruoyao <xry...@mengyan1223.wang> School of Aerospace Science and Technology, Xidian University
