> insn_propagation would previously only replace (reg:M H) with X > for some hard register H if the uses of H were also in mode M. > This patch extends it to handle simple mode punning too.
(this is from GCC commit 9d20529d94b23275885f380d155fe8671ab5353a)
Looks like this breaks m68k floating point conversion handling. Recall
that m68k has only 80-bit float hardware with regular numbered registers
(unlike 8087). Operations are done in 80-bit format, the FPU provides
fmove.d and fmove.s operations to convert between 80-bit and
32-bit/64-bit values in memory.
float naf(float, float);
int prf(char *, ...);
float sqf(float);
int func(void);
int func(void)
{
float x = naf(4.0f, 5.0f);
prf("testing sqrtf for value %a\n", (double) x);
float y = sqf(x);
prf("sqrtf is %a\n", (double) y);
return 0;
}
In this example, the return value from naf is passed as a double to prf
and also passed as a float to sqf. The value is saved on the stack
across the prf call.
In the old code, it was restored to %fp0 and then pushed as a float on
the stack to sqf:
lea prf,%a2
fmove.x %fp0,24(%sp)
jsr (%a2)
fmove.x 24(%sp),%fp0
fmove.s %fp0,-(%sp)
jsr sqf
With this patch, the low 32 bits of the value (which is garbage) gets
pushed on the stack instead:
lea prf,%a2
fmove.x %fp0,24(%sp)
jsr (%a2)
move.l 32(%sp),-(%sp)
jsr sqf
Here's the broken code:
rtx newval = to;
if (GET_MODE (x) != GET_MODE (from))
{
gcc_assert (REG_P (x) && HARD_REGISTER_P (x));
if (REG_NREGS (x) != REG_NREGS (from)
|| !REG_CAN_CHANGE_MODE_P (REGNO (x), GET_MODE (from),
GET_MODE (x)))
return false;
newval = simplify_subreg (GET_MODE (x), to, GET_MODE (from),
subreg_lowpart_offset (GET_MODE (x),
GET_MODE (from)));
if (!newval)
return false;
}
The trouble is that REG_CAN_CHANGE_MODE_P(%fp0, E_XFmode, E_SFmode)
returns true, and then simplify_subreg goes and extracts the low 32-bits
of the 80-bit value directly, skipping the conversion. This is the same
as i386, so let's adapt the i386 version of can_change_mode_class. See
attached patch, which seems to fix the bug for me.
From 862467c578545a4a7ed83767df2a181e7d845bc6 Mon Sep 17 00:00:00 2001
From: Keith Packard <kei...@keithp.com>
Date: Sat, 28 Dec 2024 23:38:31 -0800
Subject: [PATCH] m68k: TARGET_CAN_CHANGE_MODE_CLASS is false for FP regs
Just like i386, you cannot extract float or double values by using
the low bits of m68k float registers. Override this hook and check
that case.
Signed-off-by: Keith Packard <kei...@keithp.com>
---
gcc/config/m68k/m68k.cc | 22 ++++++++++++++++++++++
1 file changed, 22 insertions(+)
diff --git a/gcc/config/m68k/m68k.cc b/gcc/config/m68k/m68k.cc
index 050ac096e55..54bcfaffa72 100644
--- a/gcc/config/m68k/m68k.cc
+++ b/gcc/config/m68k/m68k.cc
@@ -200,6 +200,7 @@ static void m68k_asm_final_postscan_insn (FILE *, rtx_insn *insn, rtx [], int);
static HARD_REG_SET m68k_zero_call_used_regs (HARD_REG_SET);
static machine_mode m68k_c_mode_for_floating_type (enum tree_index);
static bool m68k_use_lra_p (void);
+static bool m68k_can_change_mode_class (machine_mode from, machine_mode to, reg_class_t rclass);
/* Initialize the GCC target structure. */
@@ -370,6 +371,9 @@ static bool m68k_use_lra_p (void);
#undef TARGET_C_MODE_FOR_FLOATING_TYPE
#define TARGET_C_MODE_FOR_FLOATING_TYPE m68k_c_mode_for_floating_type
+#undef TARGET_CAN_CHANGE_MODE_CLASS
+#define TARGET_CAN_CHANGE_MODE_CLASS m68k_can_change_mode_class
+
TARGET_GNU_ATTRIBUTES (m68k_attribute_table,
{
/* { name, min_len, max_len, decl_req, type_req, fn_type_req,
@@ -7269,4 +7273,22 @@ m68k_use_lra_p ()
return m68k_lra_p;
}
+/* Implement TARGET_CAN_CHANGE_MODE_CLASS. */
+
+static bool
+m68k_can_change_mode_class (machine_mode from,
+ machine_mode to,
+ reg_class_t rclass)
+{
+ if (from == to)
+ return true;
+
+ /* 68881 registers can't do subreg at all, as all values are reformatted
+ to extended precision. */
+ if (reg_classes_intersect_p(rclass, FP_REGS))
+ return false;
+
+ return true;
+}
+
#include "gt-m68k.h"
--
2.45.2
Here's how to reproduce the bug:
GCC configure flags:
../configure \
--mandir=/usr/share/man \
--enable-languages=c,c++,lto \
--enable-multilib \
--enable-multilib-space \
--disable-gcov \
--disable-decimal-float \
--disable-libffi \
--disable-libgomp \
--disable-libmudflap \
--disable-libquadmath \
--disable-libssp \
--disable-libstdcxx-pch \
--disable-nls \
--disable-shared \
--disable-threads \
--enable-tls \
--target=m68k-unknown-elf \
--with-system-zlib \
--with-gnu-as \
--with-gnu-ld \
--without-included-gettext \
--prefix=/opt/m68k \
--disable-libstdcxx \
--with-headers=no \
--without-newlib \
AR_FOR_TARGET=m68k-unknown-elf-ar \
AS_FOR_TARGET=m68k-unknown-elf-as \
LD_FOR_TARGET=m68k-unknown-elf-ld \
NM_FOR_TARGET=m68k-unknown-elf-nm \
OBJDUMP_FOR_TARGET=m68k-unknown-elf-objdump \
RANLIB_FOR_TARGET=m68k-unknown-elf-ranlib \
READELF_FOR_TARGET=m68k-unknown-elf-readelf \
STRIP_FOR_TARGET=m68k-unknown-elf-strip \
SED=/bin/sed \
SHELL=/bin/sh \
BASH=/bin/bash \
CONFIG_SHELL=/bin/bash
Compile flags used to build the test:
m68k-unknown-elf-gcc -O3 -S test.c
--
-keith
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