[gcc r15-1395] Make force_subreg emit nothing on failure

[Richard Sandiford via Gcc-cvs]

https://gcc.gnu.org/g:01044471ea39f9be4803c583ef2a946abc657f99

commit r15-1395-g01044471ea39f9be4803c583ef2a946abc657f99
Author: Richard Sandiford <richard.sandif...@arm.com>
Date:   Tue Jun 18 12:22:30 2024 +0100

    Make force_subreg emit nothing on failure
    
    While adding more uses of force_subreg, I realised that it should
    be more careful to emit no instructions on failure.  This kind of
    failure should be very rare, so I don't think it's a case worth
    optimising for.
    
    gcc/
            * explow.cc (force_subreg): Emit no instructions on failure.

Diff:
---
 gcc/explow.cc | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/gcc/explow.cc b/gcc/explow.cc
index f6843398c4b0..bd93c8780649 100644
--- a/gcc/explow.cc
+++ b/gcc/explow.cc
@@ -756,8 +756,12 @@ force_subreg (machine_mode outermode, rtx op,
   if (x)
     return x;
 
+  auto *start = get_last_insn ();
   op = copy_to_mode_reg (innermode, op);
-  return simplify_gen_subreg (outermode, op, innermode, byte);
+  rtx res = simplify_gen_subreg (outermode, op, innermode, byte);
+  if (!res)
+    delete_insns_since (start);
+  return res;
 }
 
 /* If X is a memory ref, copy its contents to a new temp reg and return