https://gcc.gnu.org/bugzilla/show_bug.cgi?id=89113
Bug ID: 89113
Summary: Missed stack reuse opportunity when using compound
literals
Product: gcc
Version: 8.2.1
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c
Assignee: unassigned at gcc dot gnu.org
Reporter: patrick at motec dot com.au
Target Milestone: ---
It appears as though gcc is missing a stack reuse opportunity when using
compound literals.
Consider the following example:
void f(int *);
void test1(void)
{
{
f((int []){1, 2, 3, 4});
}
{
f((int []){1, 2, 3, 4});
}
}
void test2(void)
{
{
int tmp[] = {1, 2, 3, 4};
f(tmp);
}
{
int tmp[] = {1, 2, 3, 4};
f(tmp);
}
}
The lifetimes of the compound literals in test1 should be equivalent to the
lifetimes of the tmp arrays in test2.
When compiling with gcc 8.2.1 only test2 reuses the stack locations for the
temporaries making test1 stack use higher:
% gcc -c test.c
% objdump -d test.o
0000000000000000 <test1>:
0: 55 push %rbp
1: 48 89 e5 mov %rsp,%rbp
4: 48 83 ec 30 sub $0x30,%rsp
8: 64 48 8b 04 25 28 00 mov %fs:0x28,%rax
f: 00 00
11: 48 89 45 f8 mov %rax,-0x8(%rbp)
15: 31 c0 xor %eax,%eax
17: c7 45 d0 01 00 00 00 movl $0x1,-0x30(%rbp)
1e: c7 45 d4 02 00 00 00 movl $0x2,-0x2c(%rbp)
25: c7 45 d8 03 00 00 00 movl $0x3,-0x28(%rbp)
2c: c7 45 dc 04 00 00 00 movl $0x4,-0x24(%rbp)
33: 48 8d 45 d0 lea -0x30(%rbp),%rax
37: 48 89 c7 mov %rax,%rdi
3a: e8 00 00 00 00 callq 3f <test1+0x3f>
3f: c7 45 e0 01 00 00 00 movl $0x1,-0x20(%rbp)
46: c7 45 e4 02 00 00 00 movl $0x2,-0x1c(%rbp)
4d: c7 45 e8 03 00 00 00 movl $0x3,-0x18(%rbp)
54: c7 45 ec 04 00 00 00 movl $0x4,-0x14(%rbp)
5b: 48 8d 45 e0 lea -0x20(%rbp),%rax
5f: 48 89 c7 mov %rax,%rdi
62: e8 00 00 00 00 callq 67 <test1+0x67>
67: 90 nop
68: 48 8b 45 f8 mov -0x8(%rbp),%rax
6c: 64 48 33 04 25 28 00 xor %fs:0x28,%rax
73: 00 00
75: 74 05 je 7c <test1+0x7c>
77: e8 00 00 00 00 callq 7c <test1+0x7c>
7c: c9 leaveq
7d: c3 retq
000000000000007e <test2>:
7e: 55 push %rbp
7f: 48 89 e5 mov %rsp,%rbp
82: 48 83 ec 20 sub $0x20,%rsp
86: 64 48 8b 04 25 28 00 mov %fs:0x28,%rax
8d: 00 00
8f: 48 89 45 f8 mov %rax,-0x8(%rbp)
93: 31 c0 xor %eax,%eax
95: c7 45 e0 01 00 00 00 movl $0x1,-0x20(%rbp)
9c: c7 45 e4 02 00 00 00 movl $0x2,-0x1c(%rbp)
a3: c7 45 e8 03 00 00 00 movl $0x3,-0x18(%rbp)
aa: c7 45 ec 04 00 00 00 movl $0x4,-0x14(%rbp)
b1: 48 8d 45 e0 lea -0x20(%rbp),%rax
b5: 48 89 c7 mov %rax,%rdi
b8: e8 00 00 00 00 callq bd <test2+0x3f>
bd: c7 45 e0 01 00 00 00 movl $0x1,-0x20(%rbp)
c4: c7 45 e4 02 00 00 00 movl $0x2,-0x1c(%rbp)
cb: c7 45 e8 03 00 00 00 movl $0x3,-0x18(%rbp)
d2: c7 45 ec 04 00 00 00 movl $0x4,-0x14(%rbp)
d9: 48 8d 45 e0 lea -0x20(%rbp),%rax
dd: 48 89 c7 mov %rax,%rdi
e0: e8 00 00 00 00 callq e5 <test2+0x67>
e5: 90 nop
e6: 48 8b 45 f8 mov -0x8(%rbp),%rax
ea: 64 48 33 04 25 28 00 xor %fs:0x28,%rax
f1: 00 00
f3: 74 05 je fa <test2+0x7c>
f5: e8 00 00 00 00 callq fa <test2+0x7c>
fa: c9 leaveq
fb: c3 retq
