--- a/gcc/bbb-opts.c
+++ b/gcc/bbb-opts.c
@@ -325,7 +325,17 @@
     if (GET_CODE(src) == CONST_INT && (mode == HImode || mode == QImode))
       {
 	unsigned iv = UINTVAL(src);
-	setMask(regno, iv, mode);
+	/* output_move_himode and output_move_qimode load a data register's
+	   word or byte constant in -128..127 with moveq, which writes all 32
+	   bits sign-extended, where a strict_low_part move keeps the upper
+	   ones.  A negative one (but a byte's -1, which is st) may so leave
+	   the upper bits set: keeping their old mask let opt_elim_dead_assign2
+	   delete a later zero-extension's clear as superfluous.  */
+	if (regno < 8 && INTVAL(src) >= -128 && INTVAL(src) < 0
+	    && !(mode == QImode && INTVAL(src) == -1))
+	  setMask(regno, iv, SImode);
+	else
+	  setMask(regno, iv, mode);
 	value[regno] = gen_rtx_CONST_INT(mode, 0x100000000000000LL | ((long long int ) (regno) << 32) | index);
 	used[regno] = 1 << FIRST_PSEUDO_REGISTER;
 	clearRefsByMask(1<<regno, index);