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.
---
gcc/config/m68k/m68k-protos.h | 1 +
gcc/config/m68k/m68k.cc | 25 +++++++++++++++++++++++++
gcc/config/m68k/m68k.md | 4 ++--
3 files changed, 28 insertions(+), 2 deletions(-)
diff --git a/gcc/config/m68k/m68k-protos.h b/gcc/config/m68k/m68k-protos.h
index abb8e9b061d..248414eb167 100644
--- a/gcc/config/m68k/m68k-protos.h
+++ b/gcc/config/m68k/m68k-protos.h
@@ -26,6 +26,7 @@ extern HOST_WIDE_INT m68k_initial_elimination_offset (int
from, int to);
extern void split_di (rtx[], int, rtx[], rtx[]);
extern bool valid_mov3q_const (HOST_WIDE_INT);
+extern bool check_move_simode (const rtx *);
extern const char *output_move_simode (rtx *);
extern const char *output_move_himode (rtx *);
extern const char *output_move_qimode (rtx *);
diff --git a/gcc/config/m68k/m68k.cc b/gcc/config/m68k/m68k.cc
index b6134892883..e95b83f5e18 100644
--- a/gcc/config/m68k/m68k.cc
+++ b/gcc/config/m68k/m68k.cc
@@ -3227,6 +3227,31 @@ valid_mov3q_const (HOST_WIDE_INT i)
return TARGET_ISAB && (i == -1 || IN_RANGE (i, 1, 7));
}
+/* Return true if OPERANDS[] are valid for output_move_simode.
+ In particular, if OPERANDS[1] is a MEM with PRE_DEC adressing
+ and its REG is mentioned in OPERANDS[0], it's invalid. */
+
+bool
+check_move_simode (const rtx *operands)
+{
+ rtx src = operands[1];
+ if (MEM_P (src))
+ {
+ rtx src1 = XEXP (src, 0);
+ if (GET_CODE (src1) == PRE_DEC)
+ {
+ rtx src2 = XEXP (src1, 0);
+ if (REG_P (src2))
+ {
+ rtx dst = operands[0];
+ if (reg_overlap_mentioned_p (src2, dst))
+ return false;
+ }
+ }
+ }
+ return true;
+}
+
/* Return an instruction to move CONST_INT OPERANDS[1] into OPERANDS[0].
I is the value of OPERANDS[1]. */
diff --git a/gcc/config/m68k/m68k.md b/gcc/config/m68k/m68k.md
index e77ac13cf1b..f8b6a2f79dd 100644
--- a/gcc/config/m68k/m68k.md
+++ b/gcc/config/m68k/m68k.md
@@ -993,7 +993,7 @@
;; We don't allow f-regs since fixed point cannot go in them.
[(set (match_operand:SI 0 "nonimmediate_operand" "=g,d,a<")
(match_operand:SI 1 "general_src_operand" "damSnT,n,i"))]
- "!TARGET_COLDFIRE && reload_completed"
+ "!TARGET_COLDFIRE && reload_completed && check_move_simode (operands)"
{
return output_move_simode (operands);
}
@@ -1006,7 +1006,7 @@
[(set (match_operand:SI 0 "nonimmediate_operand" "=g,d,a<")
(match_operand:SI 1 "general_src_operand" "damSKT,n,i"))]
- "!TARGET_COLDFIRE"
+ "!TARGET_COLDFIRE && check_move_simode (operands)"
{
return output_move_simode (operands);
}
--
2.54.0
