On 7/7/2026 7:31 AM, Paul Koning wrote:
On Jul 7, 2026, at 8:02 AM, Jeffrey Law <[email protected]> wrote:
On 7/7/2026 4:47 AM, Mikael Pettersson wrote:
Since gcc-15 combine can produce moves from a PRE_DEC source to a
destination that uses the same register, causing wrong code on m68k.
In particular the glibc build is broken. This adjusts the m68k
backend to reject such moves:
move.l -(%a0),(%a0,%d0.l)
and instead emit:
lea (%a0,%d0.l),%a1
move.l -(%a0),(%a1)
Bootstrapped and regression tested on m68k-linux-gnu, no regressions.
Ok for trunk? And maybe gcc-16/15 after a week?
gcc/
2026-07-07 Mikael Pettersson <[email protected]>
PR rtl-optimization/123853
* config/m68k/m68k-protos.h (check_move_simode): Declare.
* config/m68k/m68k.cc (check_move_simode): New, reject moves from
a pre-dec source mem whose reg occurs in the destination address.
* config/m68k/m68k.md (*movsi_m68k): Add check_move_simode to
condition.
(*movsi_m68k2): Likewise.
I think the new dynamic register filters are going to be the way to handle
this. The core issue is those additional uses of a auto-incremented register,
which requires looking at two operands to determine if the insn matches its
constraints. This patch just papers over the problem and it likely triggers
elsewhere in the port with some effort.
My recommendation is to wait until Pan Li's work to utilize the dynamic
register filtering lands in the RISC-V port across the board and gets a time to
shake out any additional LRA gotchas, then use it on the m68k, h8 and pdp11
which all have the same core problem.
I dealt with it in pdp11.md in a different way, using a string of constraints
to express this. It seems to work but it's rather cumbersome.
Yea. I took your idea and implemented it for the H8. As you note, it's
cumbersome and as the number of registers you have to deal with grows,
it gets worse. When we started looking at the overlap problems with
vector on RISC-V it was pretty clear that this approach wouldn't be
maintainable. The design of the dynamic filters was mean to handle the
RISC-V case, but handling the m68k, h8 and pdp11 cases was always in the
back of my mind.
Jeff
