https://gcc.gnu.org/bugzilla/show_bug.cgi?id=98335
Bug ID: 98335
Summary: [9/10/11 Regression] Poor code generation for partial
struct initialization
Product: gcc
Version: 9.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: rtl-optimization
Assignee: unassigned at gcc dot gnu.org
Reporter: mcolavita at fb dot com
Target Milestone: ---
Consider the following code:
struct Data {
char a;
int b;
};
char c;
Data val(int idx) {
return { c };
}
On x86-64 (with sizeof(char) = 1 and sizeof(int) = 4), val() can be implemented
with a single mov to %rax. With -O3, g++ produces the following inefficient
output:
xorl %eax, %eax
movb $0, -18(%rsp)
movabsq $72057594037927935, %rdx
movw %ax, -20(%rsp)
movl $0, -24(%rsp)
andq -24(%rsp), %rdx
movq %rdx, %rax
salq $8, %rax
movb c(%rip), %al
ret
Similar outputs are seen for any level of optimization above O0 on GCC 9, 10,
and 11. The bug is not present in GCC 8.
Reasonable code is generated if the second field of the struct is explicitly
initialized to a constant, either in the struct definition or the initializer.
