The attached patch is a result of discussing an S/390 issue with
"and with complement" in some cases.
https://gcc.gnu.org/ml/gcc/2016-03/msg00163.html
https://gcc.gnu.org/ml/gcc-patches/2016-04/msg01586.html
Combine would merge a ZERO_EXTEND and a SET taking the known zero
bits into account, resulting in an AND. Later on,
make_compound_operation() fails to replace that with a ZERO_EXTEND
which we get for free on S/390 but leaves the AND, eventually
resulting in two consecutive AND instructions.
The current code in make_compound_operation() that detects
opportunities for ZERO_EXTEND does not work here because it does
not take the known zero bits into account:
/* If the constant is one less than a power of two, this might be
representable by an extraction even if no shift is present.
If it doesn't end up being a ZERO_EXTEND, we will ignore it unless
we are in a COMPARE. */
else if ((i = exact_log2 (UINTVAL (XEXP (x, 1)) + 1)) >= 0)
new_rtx = make_extraction (mode,
make_compound_operation (XEXP (x, 0),
next_code),
0, NULL_RTX, i, 1, 0, in_code == COMPARE);
An attempt to use the zero bits in the above conditions resulted
in many situations that generated worse code, so the patch tries
to fix this in a more conservative way. While the effect is
completely positive on S/390, this will very likely have
unforeseeable consequences on other targets.
Bootstrapped and regression tested on s390 and s390x only at the
moment.
Ciao
Dominik ^_^ ^_^
--
Dominik Vogt
IBM Germany
gcc/ChangeLog
* combine.c (make_compound_operation): Take known zero bits into
account when checking for possible zero_extend.
>From e70e6e469200b53b3f4ae52a766cdd322a4d365d Mon Sep 17 00:00:00 2001
From: Dominik Vogt <v...@linux.vnet.ibm.com>
Date: Tue, 12 Apr 2016 09:53:46 +0100
Subject: [PATCH] Take known zero bits into account when checking
extraction.
Allows AND Insns with a const_int operand to be expressed as ZERO_EXTEND if the
operand ist a power of 2 - 1 even with the known zero bits masked out.
---
gcc/combine.c | 33 +++++++++++++++++++++++++++++++++
1 file changed, 33 insertions(+)
diff --git a/gcc/combine.c b/gcc/combine.c
index 1d0e8be..44bb1b3 100644
--- a/gcc/combine.c
+++ b/gcc/combine.c
@@ -7988,6 +7988,39 @@ make_compound_operation (rtx x, enum rtx_code in_code)
next_code),
i, NULL_RTX, 1, 1, 0, 1);
+ /* If the one operand is a paradoxical subreg of a register or memory and
+ the constant (limited to the smaller mode) has only zero bits where
+ the sub expression has known zero bits, this can be expressed as
+ a zero_extend. */
+ else if (GET_CODE (XEXP (x, 0)) == SUBREG)
+ {
+ rtx sub;
+
+ sub = XEXP (XEXP (x, 0), 0);
+ machine_mode sub_mode = GET_MODE (sub);
+ if ((REG_P (sub) || MEM_P (sub))
+ && GET_MODE_PRECISION (sub_mode) < mode_width
+ && (UINTVAL (XEXP (x, 1))
+ | (~nonzero_bits (sub, sub_mode) & GET_MODE_MASK (sub_mode))
+ ) == GET_MODE_MASK (sub_mode))
+ {
+ bool speed_p = optimize_insn_for_speed_p ();
+ rtx temp = gen_rtx_ZERO_EXTEND (mode, sub);
+ int cost_of_and;
+ int cost_of_zero_extend;
+
+ cost_of_and = rtx_cost (x, mode, in_code, 1, speed_p);
+ cost_of_zero_extend = rtx_cost (temp, mode, in_code, 1, speed_p);
+ if (cost_of_zero_extend <= cost_of_and)
+ {
+ new_rtx = make_compound_operation (sub, next_code);
+ new_rtx = make_extraction (mode, new_rtx, 0, 0,
+ GET_MODE_PRECISION (sub_mode),
+ 1, 0, in_code == COMPARE);
+ }
+ }
+ }
+
break;
case LSHIFTRT:
--
2.3.0
