https://gcc.gnu.org/bugzilla/show_bug.cgi?id=111015
Bug ID: 111015
Summary: __int128 bitfields optimized incorrectly to the 64
bit operations
Product: gcc
Version: 13.2.1
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: rtl-optimization
Assignee: unassigned at gcc dot gnu.org
Reporter: pshevchuk at pshevchuk dot com
Target Milestone: ---
godbolt: https://godbolt.org/z/r5d6ToY1z
Basically, a store of one half of a 70-bit bitfield gets completely optimized
away.
i.e. for
struct Entry {
unsigned left : 4;
unsigned right : 4;
uint128 key : KEY_BITS;
} data;
the code:
data.left = left;
data.right = right;
data.key = key & KEY_BITS_MASK;
produces the following (amd64):
andl $15, %ecx
salq $4, %rcx
andl $15, %edx
orq %rdx, %rcx
movq %rdi, %rax
salq $8, %rax
orq %rax, %rcx
movq %rcx, data(%rip)
andw $-16384, data+8(%rip)
critically, at no point is there any attempt to actually initialize data+8
The problem does not disappear if the bitfields gets moved around; it is,
however, very finicky with respect to the size of the bitfields.
-O1 -fstore-merging appears to be close to the smallest set of compilation
options at which it fails.
If you replace -O1 with the list of -O1 optimization from here:
https://gcc.gnu.org/onlinedocs/gcc/Optimize-Options.html , it will start
working correctly, so probably we have a documentation issue
