http://gcc.gnu.org/bugzilla/show_bug.cgi?id=44281
--- Comment #6 from Adam Warner <adam at consulting dot net.nz> 2011-03-04
07:22:47 UTC ---
Below is a very simple test case of an ordinary input argument to a function
being:
(a) copied to a spare register
(b) copied back from a spare register
When the input argument is:
(a) never modified; and
(b) an ordinary register (not a global register variable)
unmodified_ordinary_register_is_copied.c:
#include <stdint.h>
/* Six caller-saved registers as input arguments */
#define CALLER_SAVED uint64_t REG0, uint64_t REG1, uint64_t REG2, \
uint64_t REG3, uint64_t REG4, uint64_t REG5
typedef void (*fn_t)(CALLER_SAVED);
/* Six callee-saved registers as global register variables */
register uint64_t REG6 __asm__("rbx");
register fn_t *REG7 __asm__("rbp");
register uint64_t REG8 __asm__("r12");
register uint64_t REG9 __asm__("r13");
register uint64_t REG10 __asm__("r14");
register uint64_t REG11 __asm__("r15");
/* Free general purpose registers are RSP, RAX, R10 and R11 */
void optimal_code_generation(CALLER_SAVED) {
fn_t next=REG7[1];
next(REG0, REG1, REG2, REG3, REG4, REG5);
}
void unmodified_input_arg_is_copied(CALLER_SAVED) {
fn_t next=REG7[1];
++REG7;
next(REG0, REG1, REG2, REG3, REG4, REG5);
}
int main() {
return 0;
}
gcc-4.5 generates optimal code for both functions:
$ gcc-4.5 -O3 unmodified_ordinary_register_is_copied.c && objdump -d -m
i386:x86-64 a.out|less
...
00000000004004a0 <optimal_code_generation>:
4004a0: 48 8b 45 08 mov 0x8(%rbp),%rax
4004a4: ff e0 jmpq *%rax
...
00000000004004b0 <unmodified_input_arg_is_copied>:
4004b0: 48 8b 45 08 mov 0x8(%rbp),%rax
4004b4: 48 83 c5 08 add $0x8,%rbp
4004b8: ff e0 jmpq *%rax
...
Compare with GCC 4.6:
$ gcc-4.6 --version
gcc-4.6 (Debian 4.6-20110227-1) 4.6.0 20110227 (experimental) [trunk revision
170543]
...
$ gcc-4.6 -O3 unmodified_ordinary_register_is_copied.c && objdump -d -m
i386:x86-64 a.out|less
...
00000000004004a0 <optimal_code_generation>:
4004a0: 48 8b 45 08 mov 0x8(%rbp),%rax
4004a4: ff e0 jmpq *%rax
...
00000000004004b0 <unmodified_input_arg_is_copied>:
4004b0: 49 89 fa mov %rdi,%r10
4004b3: 48 8b 45 08 mov 0x8(%rbp),%rax
4004b7: 48 8d 6d 08 lea 0x8(%rbp),%rbp
4004bb: 4c 89 d7 mov %r10,%rdi
4004be: ff e0 jmpq *%rax
...
According to the Linux x86-64 ABI %rdi is the first argument passed to the
functions. For some reason this is being copied to %r10 before being copied
back from %r10 to %rdi. At no stage is %rdi modified.
(Minor aside:
lea 0x8(%rbp),%rbp has also replaced add $0x8,%rbp. My Intel Core 2 hardware
can execute a maximum of one LEA instruction per clock cycle compared to three
ADD instructions per clock cycle. If I add -march=core2 -mtune=core2 the code
generation becomes:
00000000004004b0 <unmodified_input_arg_is_copied>:
4004b0: 48 8b 45 08 mov 0x8(%rbp),%rax
4004b4: 48 8d 6d 08 lea 0x8(%rbp),%rbp
4004b8: 49 89 fa mov %rdi,%r10
4004bb: 4c 89 d7 mov %r10,%rdi
4004be: ff e0 jmpq *%rax
)
This bizarre register copying goes away if I comment out one of the six global
register variables (i.e. five callee-saved global register variables instead of
six). For some reason GCC 4.6 cannot generate sensible code with %rsp, %rax,
%r10 and %r11 available---but can generate sensible code when an additional
register (%rbx, %r12, %r13, %r14 or %r15) is available.
