Hello all,
I tracing bug in GCC for Coldfire target, but I end in dead water and
I need some help from real experts :).
Both gcc 4.4 and 4.3 have same problem
GCC miscompile this small test case.
//-----------------------------------------------------------------
//m68k-elf-gcc -save-temps -da -fdump-tree-all -fdump-ipa-all -c test.c -o
test.o
void dummy(char *arg);
static void test1(void)
{
char tmp[2] = "0";
}
void test2(void)
{
dummy("0");
}
//------------------------------------------------------------------
The file is compiled to:
#NO_APP
.file "test.c"
.section .rodata
.LC0:
.string "0"
.text
.align 2
.type test1, @function
test1:
link.w %fp,#-4
lea .LC0,%a0
move.w (%a0),-2(%fp)
unlk %fp
rts
.size test1, .-test1
.align 2
.globl test2
.type test2, @function
test2:
link.w %fp,#0
move.l %a0,-(%sp) <-- note: a0 is used uninitialized here
jsr dummy
addq.l #4,%sp
unlk %fp
rts
.size test2, .-test2
.ident "GCC: (GNU) 4.4.0 20081031 (experimental)"
And relevant part of RTL after expand pass:
;; Function test1 (test1)
;; Generating RTL for gimple basic block 2
;; tmp ={v} "0";
(insn 5 4 0 test.c:8 (set (mem/s/c:HI (plus:SI (reg/f:SI 26 virtual-stack-vars)
(const_int -2 [0xfffffffe])) [0 tmp+0 S2 A16])
(mem/s:HI (symbol_ref/f:SI ("*.LC0") [flags 0x2] <string_cst
0x7fdf7700>) [0 S2 A8])) -1 (nil))
;; Function test2 (test2)
;; Generating RTL for gimple basic block 2
;; dummy (&"0"[0]);
<--- !!!! bad insn here -----
(insn 5 4 6 test.c:15 (set (mem/f/i:SI (pre_dec:SI (reg/f:SI 15 %sp)) [0 S4
A16])
(reg:SI 8 %a0)) -1 (nil))
<------------------------------
(call_insn 6 5 7 test.c:15 (call (mem:QI (symbol_ref:SI ("dummy") [flags 0x41]
<function_decl 0x7fdd4580 dummy>) [0 S1 A8])
(const_int 4 [0x4])) -1 (nil)
(nil))
(insn 7 6 0 test.c:15 (set (reg/f:SI 15 %sp)
(plus:SI (reg/f:SI 15 %sp)
(const_int 4 [0x4]))) -1 (nil))
After some debugging, I found cause of this bug, but proper solution is
not clear for me. When "char tmp[2] = "0";" is compiled, the function
"output_constant_def"
is called and proper insn is stored into cache:
(gdb) call debug_rtx(desc->rtl)
(mem/s:HI (symbol_ref/f:SI ("*.LC0") [flags 0x2] <string_cst 0x7fdf7700>) [0
S2 A8])
Later, in ira pass, this insn is spitted to this (Coldfire has no memory to
memory move):
Reloads for insn # 5
Reload 0: reload_in (SI) = (symbol_ref/f:SI ("*.LC0") [flags 0x2] <string_cst
0x7fdf7700>)
ADDR_REGS, RELOAD_FOR_INPUT (opnum = 1), inc by 2
reload_in_reg: (symbol_ref/f:SI ("*.LC0") [flags 0x2] <string_cst
0x7fdf7700>)
reload_reg_rtx: (reg:SI 8 %a0)
...
(note 2 3 14 2 NOTE_INSN_FUNCTION_BEG)
(insn 14 2 5 2 test.c:7 (set (reg:SI 8 %a0)
(symbol_ref/f:SI ("*.LC0") [flags 0x2] <string_cst 0x7fdf7700>)) 38
{*movsi_cf} (nil))
(insn 5 14 13 2 test.c:7 (set (mem/s/c:HI (plus:SI (reg/f:SI 14 %a6)
(const_int -2 [0xfffffffe])) [0 tmp+0 S2 A16])
(mem/s:HI (reg:SI 8 %a0) [0 S2 A8])) 41 {*m68k.md:906} (nil))
;; End of basic block 2 -> ( 1)
;; lr out 14 [%a6] 15 [%sp] 24 [%argptr]
The first insn is newly allocated, but second one overwrites original insn.
>From local point of view, this is still OK, but this corrupt the
>output_constant_def
cache (cache holds pointer to overwritten insn. So every
next access to "0" constat returns
(mem/s:HI (reg:SI 8 %a0) [0 S2 A8])) 41 {*m68k.md:906} (nil))
and not
(symbol_ref/f:SI ("*.LC0") [flags 0x2] <string_cst 0x7fdf7700>)) 38
{*movsi_cf} (nil))
But how to fix this? By my mean, the cache must hold own immutable copy of insn.
But im not gcc expert, so I need help with proper solution. Is this patch OK?
Any other solution?
--- varasm.c.orig 2008-11-14 18:04:27.693643900 +0100
+++ varasm.c 2008-11-14 17:58:06.522748300 +0100
@@ -3245,7 +3245,7 @@
}
maybe_output_constant_def_contents (desc, defer);
- return desc->rtl;
+ return copy_rtx (desc->rtl);
}
/* Subroutine of output_constant_def: Decide whether or not we need to
---------------------------------------------
Moreover, this can be common problem on more places (at least at
gen_rtx_CONST_INT).
Ohh, and sorry for my english.
Many thanks
Michal Meloun
