From: Richard Sandiford <richard.sandiford@arm.com>
Date: Mon, 21 Jul 2025 14:41:01 +0000 (+0100)
Subject: ext-dce: Fix subreg_lsb is_constant assumption (2)
X-Git-Url: http://git.ipfire.org/cgi-bin/gitweb.cgi?a=commitdiff_plain;h=0b6038c17cabdbc8b9bfd0d13e2dd6f74db78734;p=thirdparty%2Fgcc.git

ext-dce: Fix subreg_lsb is_constant assumption (2)

This patch fixes another instance of the problem described in the
cover note for g:bf3037e923e9f91d93ab64bdf73a37f64f659fb9.

gcc/
	* ext-dce.cc (ext_dce_process_uses): Apply is_constant directly
	to the subreg_lsb.

(cherry picked from commit bddc41e290113dd9160b01f2fdf925a1876c8ee0)
---

diff --git a/gcc/ext-dce.cc b/gcc/ext-dce.cc
index e7635fb7a39..67ec92a4287 100644
--- a/gcc/ext-dce.cc
+++ b/gcc/ext-dce.cc
@@ -757,7 +757,7 @@ ext_dce_process_uses (rtx_insn *insn, rtx obj,
 		     and process the inner object.  */
 		  if (paradoxical_subreg_p (y))
 		    y = XEXP (y, 0);
-		  else if (SUBREG_P (y) && SUBREG_BYTE (y).is_constant ())
+		  else if (SUBREG_P (y) && subreg_lsb (y).is_constant (&bit))
 		    {
 		      /* If !TRULY_NOOP_TRUNCATION_MODES_P, the mode
 			 change performed by Y would normally need to be a
@@ -774,8 +774,6 @@ ext_dce_process_uses (rtx_insn *insn, rtx obj,
 				    GET_MODE (SUBREG_REG (y))))))
 			break;
 
-		      bit = subreg_lsb (y).to_constant ();
-
 		      /* If this is a wide object (more bits than we can fit
 			 in a HOST_WIDE_INT), then just break from the SET
 			 context.   That will cause the iterator to walk down