On Fri, 14 Jul 2023, Uros Bizjak wrote:
> cprop1 pass does not consider paradoxical subreg and for (insn 22) claims
> that it equals 8 elements of HImodeby setting REG_EQUAL note:
>
> (insn 21 19 22 4 (set (reg:V4QI 98)
> (mem/u/c:V4QI (symbol_ref/u:DI ("*.LC1") [flags 0x2]) [0 S4
> A32])) "pr110206.c":12:42 1530 {*movv4qi_internal}
> (expr_list:REG_EQUAL (const_vector:V4QI [
> (const_int -52 [0xffffffffffffffcc]) repeated x4
> ])
> (nil)))
> (insn 22 21 23 4 (set (reg:V8HI 100)
> (zero_extend:V8HI (vec_select:V8QI (subreg:V16QI (reg:V4QI 98) 0)
> (parallel [
> (const_int 0 [0])
> (const_int 1 [0x1])
> (const_int 2 [0x2])
> (const_int 3 [0x3])
> (const_int 4 [0x4])
> (const_int 5 [0x5])
> (const_int 6 [0x6])
> (const_int 7 [0x7])
> ])))) "pr110206.c":12:42 7471
> {sse4_1_zero_extendv8qiv8hi2}
> (expr_list:REG_EQUAL (const_vector:V8HI [
> (const_int 204 [0xcc]) repeated x8
> ])
> (expr_list:REG_DEAD (reg:V4QI 98)
> (nil))))
>
> We rely on the "undefined" vals to have a specific value (from the earlier
> REG_EQUAL note) but actual code generation doesn't ensure this (it doesn't
> need to). That said, the issue isn't the constant folding per-se but that
> we do not actually constant fold but register an equality that doesn't hold.
>
> PR target/110206
>
> gcc/ChangeLog:
>
> * fwprop.cc (contains_paradoxical_subreg_p): Move to ...
> * rtlanal.cc (contains_paradoxical_subreg_p): ... here.
> * rtlanal.h (contains_paradoxical_subreg_p): Add prototype.
> * cprop.cc (try_replace_reg): Do not set REG_EQUAL note
> when the original source contains a paradoxical subreg.
>
> gcc/testsuite/ChangeLog:
>
> * gcc.dg/torture/pr110206.c: New test.
>
> Bootstrapped and regression tested on x86_64-linux-gnu {,-m32}.
>
> OK for mainline and backports?
OK.
I think the testcase can also run on other targets if you add
dg-additional-options "-w -Wno-psabi", all generic vector ops
should be lowered if not supported.
Richard.
