Ok, here's where it's getting expanded:
#12 0x081e2805 in convert_modes (mode=PSImode, oldmode=HImode, x=0xb7f63450,
unsignedp=1)
at ../../gcc/gcc/expr.c:859
#13 0x0831f83a in expand_binop_directly (mode=945, binoptab=0x8824de0,
op0=0xb7f63410,
op1=0xb7f63450, target=0xb7f63440, unsignedp=<value optimized out>,
methods=OPTAB_LIB_WIDEN,
last=0xb7ef16c0) at ../../gcc/gcc/optabs.c:1488
#14 0x0831aa62 in expand_binop (mode=PSImode, binoptab=0x8824de0,
op0=0xb7f63410, op1=0xb7f63450,
target=0xb7f63440, unsignedp=1, methods=OPTAB_LIB_WIDEN) at
../../gcc/gcc/optabs.c:1601
#15 0x0821cb22 in expand_expr_real_1 (exp=0xb7ef160c, target=0xb7f63440,
tmode=PSImode,
modifier=EXPAND_NORMAL, alt_rtl=0x0) at ../../gcc/gcc/expr.c:9452
At #15 the type of the target is used to decide if the math is signed
or unsigned, so adding a signed HI to an unsigned PSI results in a
zero extend.
The patch you added was for the multiply-and-add case, such as "int *p
+= 1" but it's skipped if there's no multiply (char *p).
