https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102758
Andrew Pinski <pinskia at gcc dot gnu.org> changed:
What |Removed |Added
----------------------------------------------------------------------------
Severity|normal |minor
Keywords|needs-bisection |
Summary|[12 Regression] Failure to |[12 Regression] Failure to
|use registers optimally |use registers optimally
|with return values (2 |with return values (2
|operands related) |operands related and
| |subreg)
--- Comment #2 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
So the difference in the IR happens from forwprop.
before RA we have this for the old compiler:
(insn 9 8 14 2 (set (reg:V8HI 89)
(plus:V8HI (reg:V8HI 90)
(subreg:V8HI (reg:V2DI 91) 0))) "/app/example.cpp":8:29 3418
{*addv8hi3}
(expr_list:REG_DEAD (reg:V2DI 91)
(expr_list:REG_DEAD (reg:V8HI 90)
(nil))))
(insn 14 9 15 2 (set (reg/i:V2DI 20 xmm0)
(subreg:V2DI (reg:V8HI 89) 0)) "/app/example.cpp":9:1 1439
{movv2di_internal}
(expr_list:REG_DEAD (reg:V8HI 89)
(nil)))
With the new compile we have:
(insn 10 9 14 2 (set (subreg:V8HI (reg:V2DI 84 [ <retval> ]) 0)
(plus:V8HI (reg:V8HI 90)
(subreg:V8HI (reg:V2DI 91) 0))) "/app/example.cpp":8:41 5787
{*addv8hi3}
(expr_list:REG_DEAD (reg:V8HI 90)
(expr_list:REG_DEAD (reg:V2DI 91)
(nil))))
(insn 14 10 15 2 (set (reg/i:V2DI 20 xmm0)
(reg:V2DI 84 [ <retval> ])) "/app/example.cpp":9:1 1634
{movv2di_internal}
(expr_list:REG_DEAD (reg:V2DI 84 [ <retval> ])
(nil)))
This now confuses the register allocator.
So this is only an issue with return because of the hard register being
involved. So this is a minor issue and not much of a problem unless you are
doing microbenchmarking.
