https://gcc.gnu.org/bugzilla/show_bug.cgi?id=99997
Bug ID: 99997
Summary: Missed optimisation with -Os
Product: gcc
Version: 10.2.1
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c
Assignee: unassigned at gcc dot gnu.org
Reporter: dhowells at redhat dot com
Target Milestone: ---
Created attachment 50538
--> https://gcc.gnu.org/bugzilla/attachment.cgi?id=50538&action=edit
Test source
Using the Fedora 33 x86_64 compiler:
gcc version 10.2.1 20201125 (Red Hat 10.2.1-9) (GCC)
Building the following (see also attached file):
typedef _Bool bool;
#define __always_inline inline __attribute__((__always_inline__))
enum { PG_head = 16 };
struct page {
unsigned long flags;
unsigned long compound_head; /* Bit zero is set */
};
static inline bool constant_test_bit(int nr, const void *addr)
{
const unsigned int *p = (const unsigned int *)addr;
return ((1UL << (nr & 31)) & (p[nr >> 5])) != 0;
}
static __always_inline bool PageTail(struct page *page)
{
return page->compound_head & 1;
}
static __always_inline bool PageCompound(struct page *page)
{
return constant_test_bit(PG_head, &page->flags) || PageTail(page);
}
bool PageTransCompound(struct page *page)
{
return PageCompound(page);
}
with "gcc -Os" I get the following assembly:
PageTransCompound:
.LFB3:
.cfi_startproc
movl (%rdi), %edx
movl $1, %eax
btl $16, %edx
jc .L2
movq 8(%rdi), %rax
andl $1, %eax
.L2:
andl $1, %eax
ret
.cfi_endproc
There are two consecutive identical ANDL instructions, one of which is
superfluous. The compile could eliminate the one that's immediately prior to
the .L2 instruction.
