Add force_lowpart_subreg

optabs had a local function called lowpart_subreg_maybe_copy
that is very similar to the lowpart version of force_subreg.
This patch adds a force_lowpart_subreg wrapper around
force_subreg and uses it in optabs.cc.

The only difference between the old and new functions is that
the old one asserted success while the new one doesn't.
It's common not to assert elsewhere when taking subregs;
normally a null result is enough.

Later patches will make more use of the new function.

gcc/
	* explow.h (force_lowpart_subreg): Declare.
	* explow.cc (force_lowpart_subreg): New function.
	* optabs.cc (lowpart_subreg_maybe_copy): Delete.
	(expand_absneg_bit): Use force_lowpart_subreg instead of
	lowpart_subreg_maybe_copy.
	(expand_copysign_bit): Likewise.

3 files changed, 17 insertions, 22 deletions

diff --git a/gcc/explow.cc b/gcc/explow.cc
index bd93c87..2a91cf7 100644
--- a/gcc/explow.cc
+++ b/gcc/explow.cc
@@ -764,6 +764,20 @@ force_subreg (machine_mode outermode, rtx op,
   return res;
 }
 
+/* Try to return an rvalue expression for the OUTERMODE lowpart of OP,
+   which has mode INNERMODE.  Allow OP to be forced into a new register
+   if necessary.
+
+   Return null on failure.  */
+
+rtx
+force_lowpart_subreg (machine_mode outermode, rtx op,
+		      machine_mode innermode)
+{
+  auto byte = subreg_lowpart_offset (outermode, innermode);
+  return force_subreg (outermode, op, innermode, byte);
+}
+
 /* If X is a memory ref, copy its contents to a new temp reg and return
    that reg.  Otherwise, return X.  */
 
diff --git a/gcc/explow.h b/gcc/explow.h
index cbd1fcb..dd65464 100644
--- a/gcc/explow.h
+++ b/gcc/explow.h
@@ -43,6 +43,7 @@ extern rtx copy_to_suggested_reg (rtx, rtx, machine_mode);
 extern rtx force_reg (machine_mode, rtx);
 
 extern rtx force_subreg (machine_mode, rtx, machine_mode, poly_uint64);
+extern rtx force_lowpart_subreg (machine_mode, rtx, machine_mode);
 
 /* Return given rtx, copied into a new temp reg if it was in memory.  */
 extern rtx force_not_mem (rtx);
diff --git a/gcc/optabs.cc b/gcc/optabs.cc
index c54d275..d569742 100644
--- a/gcc/optabs.cc
+++ b/gcc/optabs.cc
@@ -3096,26 +3096,6 @@ expand_ffs (scalar_int_mode mode, rtx op0, rtx target)
   return 0;
 }
 
-/* Extract the OMODE lowpart from VAL, which has IMODE.  Under certain
-   conditions, VAL may already be a SUBREG against which we cannot generate
-   a further SUBREG.  In this case, we expect forcing the value into a
-   register will work around the situation.  */
-
-static rtx
-lowpart_subreg_maybe_copy (machine_mode omode, rtx val,
-			   machine_mode imode)
-{
-  rtx ret;
-  ret = lowpart_subreg (omode, val, imode);
-  if (ret == NULL)
-    {
-      val = force_reg (imode, val);
-      ret = lowpart_subreg (omode, val, imode);
-      gcc_assert (ret != NULL);
-    }
-  return ret;
-}
-
 /* Expand a floating point absolute value or negation operation via a
    logical operation on the sign bit.  */
 
@@ -3204,7 +3184,7 @@ expand_absneg_bit (enum rtx_code code, scalar_float_mode mode,
 			   gen_lowpart (imode, op0),
 			   immed_wide_int_const (mask, imode),
 		           gen_lowpart (imode, target), 1, OPTAB_LIB_WIDEN);
-      target = lowpart_subreg_maybe_copy (mode, temp, imode);
+      target = force_lowpart_subreg (mode, temp, imode);
 
       set_dst_reg_note (get_last_insn (), REG_EQUAL,
 			gen_rtx_fmt_e (code, mode, copy_rtx (op0)),
@@ -4043,7 +4023,7 @@ expand_copysign_bit (scalar_float_mode mode, rtx op0, rtx op1, rtx target,
 
       temp = expand_binop (imode, ior_optab, op0, op1,
 			   gen_lowpart (imode, target), 1, OPTAB_LIB_WIDEN);
-      target = lowpart_subreg_maybe_copy (mode, temp, imode);
+      target = force_lowpart_subreg (mode, temp, imode);
     }
 
   return target;