https://gcc.gnu.org/bugzilla/show_bug.cgi?id=115877
Xi Ruoyao <xry111 at gcc dot gnu.org> changed:
What |Removed |Added
----------------------------------------------------------------------------
See Also| |https://gcc.gnu.org/bugzill
| |a/show_bug.cgi?id=116066
--- Comment #19 from Xi Ruoyao <xry111 at gcc dot gnu.org> ---
(In reply to Xi Ruoyao from comment #14)
> When I'm testing a LoongArch patch, ext_dce seems removing a zero extension
> in libcpp/expr.cc, causing stage 2 GCC miscompiled. Before ext_dce:
>
> (insn 110 108 115 14 (set (reg:DI 91 [ num$17 ])
> (zero_extend:DI (subreg:QI (reg:SI 134) 0)))
> "../../gcc/libcpp/expr.cc":1968:17 discrim 6 129 {zero_extendqidi2}
> (expr_list:REG_DEAD (reg:SI 134)
> (nil)))
>
> (insn 3472 1254 1255 145 (set (reg:DI 1530)
> (reg:DI 294 [ num$17 ])) "../../gcc/libcpp/expr.cc":1565:33 158
> {*movdi_64bit}
> (nil))
>
> (insn 1259 1258 134 145 (set (reg:DI 329 [ prephitmp_444 ])
> (zero_extend:DI (subreg/s/v:QI (reg:DI 1530) 0)))
> "../../gcc/libcpp/expr.cc":1681:22 129 {zero_extendqidi2}
> (expr_list:REG_DEAD (reg:DI 1530)
> (nil)))
>
> But during ext_dce:
>
> Processing insn:
> 110: r91:DI=zero_extend(r134:SI#0)
> REG_DEAD r134:SI
> Trying to simplify pattern:
> (zero_extend:DI (subreg:QI (reg:SI 134) 0))
> rescanning insn with uid = 110.
> Successfully transformed to:
> (subreg:DI (reg:SI 134) 0)
>
> So insn 110 becomes:
>
> (insn 110 108 115 14 (set (reg:DI 91 [ num$17 ])
> (subreg:DI (reg:SI 134) 0)) "../../gcc/libcpp/expr.cc":1968:17
> discrim 6 158 {*movdi_64bit}
> (expr_list:REG_DEAD (reg:SI 134)
> (nil)))
>
> But insn 3472 and 1259 are not changed (i.e. in 1259 the
> SUBREG_PROMOTED_VAR_P, printed as "/s" isn't removed). Subsequently the
> combine pass optimizes 1259 to:
>
> (insn 1259 1258 134 145 (set (reg:DI 329 [ prephitmp_444 ])
> (reg:DI 294 [ num$17 ])) "../../gcc/libcpp/expr.cc":1681:22 158
> {*movdi_64bit}
> (expr_list:REG_DEAD (reg:DI 294 [ num$17 ])
> (nil)))
>
> And now we are doomed. Either insn 110 shouldn't be changed or
> SUBREG_PROMOTED_VAR_P in 1259 should be removed. I see some code in
> ext-dce.cc removing SUBREG_PROMOTED_VAR_P but it seems this case isn't
> caught.
>
> I tried to make a reduced test case but failed.
