https://gcc.gnu.org/bugzilla/show_bug.cgi?id=106818
Andrew Pinski <pinskia at gcc dot gnu.org> changed:
What |Removed |Added
----------------------------------------------------------------------------
Last reconfirmed| |2022-10-21
Status|UNCONFIRMED |NEW
Summary|code is genereated |riscv produces bad low_sum
|differently with or without |while doing expansion of
|'extern' |strict aligned stores/load
Component|middle-end |target
Ever confirmed|0 |1
Keywords|missed-optimization |wrong-code
--- Comment #11 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
So on aarch64 with -O3 -mstrict-align we produce:
adrp x1, array
add x0, x1, :lo12:array
mov w2, 10
strb w2, [x1, #:lo12:array]
strb wzr, [x0, 1]
strb wzr, [x0, 2]
strb wzr, [x0, 3]
PowerPC has a similar output as aarch64 here too.
Which means this is a target issue where low_sum is being combined with the add
when it should not be.
Confirmed for riscv.
It happens at expand time:
(insn 15 14 16 (set (mem/c:QI (lo_sum:DI (reg:DI 72)
(symbol_ref:DI ("array") [flags 0xc4] <var_decl 0x7f347a4c51b0
array>)) [1 MEM[(struct sss_t *)&array].i+0 S1 A8])
(reg:QI 78)) "/app/example.cpp":10:10 -1
(nil))
(insn 16 15 17 (set (reg:DI 80)
(zero_extend:DI (mem/c:QI (lo_sum:DI (reg:DI 72)
(const:DI (plus:DI (symbol_ref:DI ("array") [flags 0xc4]
<var_decl 0x7f347a4c51b0 array>)
(const_int 1 [0x1])))) [1 MEM[(struct sss_t
*)&array].i+1 S1 A8]))) "/app/example.cpp":10:10 -1
(nil))
Note the alignment is causing the difference in doing the expansion or one
store but then the wrong code is due to the backend expansion of the address
incorrectly.
ARM64 (and powerpc) expansion produces:
(insn 18 17 19 (set (reg:QI 101)
(mem/c:QI (plus:DI (reg/f:DI 92)
(const_int 1 [0x1])) [1 MEM[(struct sss_t *)&array].i+1 S1
A8])) "/app/example.cpp":10:10 -1
(nil))
Notice the 92 rather than the low_sum part.
