zen% /opt/gcc-4.5.0/bin/gcc -v
Using built-in specs.
COLLECT_GCC=/opt/gcc-4.5.0/bin/gcc
COLLECT_LTO_WRAPPER=/opt/gcc-4.5.0/libexec/gcc/i686-pc-linux-gnu/4.5.0/lto-wrapper
Target: i686-pc-linux-gnu
Configured with: /src/package/lang/other/gcc-4.5.0/configure
--prefix=/opt/gcc-4.5.0 --with-gmp=/opt/gmp-4.2.2 --with-mpfr=/opt/mpfr-2.4.1
--with-mpc=/opt/mpc-0.8.1
Thread model: posix
gcc version 4.5.0 (GCC)
zen%
Compiling with an appropriate LD_LIBRARY_PATH and "/opt/gcc-4.5.0/bin/gcc -o
gcctest.s -S -O3 gcctest.c", the assembler produced for the following C code
includes unnecessary stack manipulation instructions. I suspect this is
x86-specific preamble/postamble generation being confused by the tail-call
optimization.
zen% cat gcctest.c
typedef int (f2_t)(int pi1, int pi2);
typedef int (f3_t)(f2_t* pf2, int pi1, int pi2);
typedef struct { f2_t* sf2; f3_t* sf3; } cmp_t;
int* gip;
int f2(int pi1, int pi2) { return 0; }
int f3(f2_t* pf2, int pi1, int pi2) {
return pf2(gip[pi1], gip[pi2]);
}
int main(void) {
cmp_t ct = { &f2, &f3 };
return ct.sf3(ct.sf2, 0, 0);
}
zen%
The assembler emitted for f3() is:
.p2align 4,,15
.globl f3
.type f3, @function
f3:
pushl %ebp
movl %esp, %ebp
pushl %ebx
subl $4, %esp ; this is not needed
movl gip, %eax
movl 16(%ebp), %ebx
movl 12(%ebp), %ecx
movl 8(%ebp), %edx
movl (%eax,%ebx,4), %ebx
movl %ebx, 12(%ebp)
movl (%eax,%ecx,4), %eax
movl %eax, 8(%ebp)
addl $4, %esp ; this is not needed
popl %ebx
popl %ebp
jmp *%edx
.size f3, .-f3
I spotted this initially with gcc-4.4.3, and have just verified that the same
code is emitted for 4.5.0.
Given that in similar code without a tail-call the stack manipulation is
completely elided, this could maybe be classed as a bug. However, the code as
generated is not incorrect.
--
Summary: Suboptimal x86 pre/postamble emitted
Product: gcc
Version: 4.5.0
Status: UNCONFIRMED
Severity: enhancement
Priority: P3
Component: other
AssignedTo: unassigned at gcc dot gnu dot org
ReportedBy: hv at crypt dot org
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=43992
