[PATCH 1/8] Make force_subreg emit nothing on failure

[Richard Sandiford]

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

diff --git a/gcc/explow.cc b/gcc/explow.cc
index f6843398c4b..bd93c878064 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
-- 
2.25.1