http://gcc.gnu.org/bugzilla/show_bug.cgi?id=57264
Bug ID: 57264
Summary: cld not emitted when string instructions used, and
'-mcld' on command line
Product: gcc
Version: unknown
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c
Assignee: unassigned at gcc dot gnu.org
Reporter: thutt at vmware dot com
A defect has been found in several versions of gcc with respect to the
'-mcld' option.
-mcld
This option instructs GCC to emit a "cld" instruction in the
prologue of functions that use string instructions. String
instructions depend on the DF flag to select between
autoincrement or autodecrement mode. While the ABI specifies the
DF flag to be cleared on function entry, some operating systems
violate this specification by not clearing the DF flag in their
exception dispatchers. The exception handler can be invoked with
the DF flag set which leads to wrong direction mode, when string
instructions are used. This option can be enabled by default on
32-bit x86 targets by configuring GCC with the --enable-cld
configure option. Generation of "cld" instructions can be
suppressed with the -mno-cld compiler option in this case.
Consider the following distillation of the issue (cld.c):
void test(int x, int **pp)
{
while (x) {
int *ip = *pp;
int *op = *pp;
while (*ip) {
int v = *ip++;
*op++ = v + 1;
}
}
}
To demonstrate the issue:
gcc -c -mcld -O1 -m64 -o cld.o cld.c
objdump --disassemble --reloc cld.o
An example failure case:
gcc (Ubuntu/Linaro 4.6.3-1ubuntu5) 4.6.3
Copyright (C) 2011 Free Software Foundation, Inc.
This is free software; see the source for copying conditions. There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
0000000000000000 <test>:
0: 85 ff test %edi,%edi
2: 74 15 je 19 <test+0x19>
4: 48 8b 3e mov (%rsi),%rdi
7: 8b 07 mov (%rdi),%eax
9: 85 c0 test %eax,%eax
b: 74 f7 je 4 <test+0x4>
d: 83 c0 01 add $0x1,%eax
10: ab stos %eax,%es:(%rdi)
11: 8b 07 mov (%rdi),%eax
13: 85 c0 test %eax,%eax
15: 74 ed je 4 <test+0x4>
17: eb f4 jmp d <test+0xd>
19: f3 c3 repz retq
For the above sample code, local experimentation has shown than 4.5,
4.6 & 4.7 emit a string instruction without a 'cld' in the prologue.
Versions 4.4 & 4.8 do not emit a string instruction.
Most perturbations to the above sample code will change the output so
that no string instruction is emitted when using the defective
versions.
It is expected that a 'cld' instruction will be generated in the
function prolong whenever any string instruction is emitted by gcc when
the '-mcld' option has been provided.
