https://gcc.gnu.org/bugzilla/show_bug.cgi?id=88765
Bug ID: 88765
Summary: powerpc64le-linux-gnu sub-optimal code generation for
builtin atomic ops
Product: gcc
Version: 8.2.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: target
Assignee: unassigned at gcc dot gnu.org
Reporter: npiggin at gmail dot com
Target Milestone: ---
Target: powerpc64le-linux-gnu
gcc version 8.2.0 (Debian 8.2.0-4)
Linux uses a lot of non-trivial operations, and implementing them with
compare_exchange results in sub-optimal code generation. A common one is
add_unless_zero, which is commonly used with RCU to take a reference count, or
fail if the last reference had already gone (which is a very rare case).
---
#include <stdbool.h>
bool add_unless_zero(unsigned long *mem, unsigned long inc)
{
unsigned long val;
do {
val = __atomic_load_n(mem, __ATOMIC_RELAXED);
if (__builtin_expect(val == 0, false))
return false;
} while (__builtin_expect(!__atomic_compare_exchange_n(mem,
&val, val + inc, true,
__ATOMIC_RELAXED, __ATOMIC_RELAXED), false));
return true;
}
---
This compiles to:
add_unless_zero:
.L4:
ld 10,0(3)
cmpdi 7,10,0
add 8,10,4
beq 7,.L5
ldarx 9,0,3
cmpd 0,9,10
bne 0,.L3
stdcx. 8,0,3
.L3:
bne 0,.L4
li 3,1
blr
.L5:
li 3,0
blr
Better would be
add_unless_zero:
.L4:
ldarx 9,0,3
cmpdi 0,9,0
add 9,9,4
bne 0,.L5
stdcx. 8,0,3
bne 0,.L4
li 3,1
blr
.L5:
li 3,0
blr
Using extended inline asm to implement these is an adequate workaround.
Unfortunately that does not work on 128 bit powerpc because no register pair
constraint, and much worse code generation with builtins. Changing types to
__int128_t gives a bad result:
add_unless_zero:
lq 10,0(3)
mr 6,3
or. 9,10,11
addc 3,11,4
mr 7,11
adde 11,10,5
beq 0,.L16
std 28,-32(1)
std 29,-24(1)
std 30,-16(1)
std 31,-8(1)
.L12:
lqarx 28,0,6
xor 9,29,7
xor 10,28,10
or. 9,9,10
bne 0,.L4
mr 30,11
mr 31,3
stqcx. 30,0,6
.L4:
mfcr 3,128
rlwinm 3,3,3,1
bne 0,.L17
.L2:
ld 28,-32(1)
ld 29,-24(1)
ld 30,-16(1)
ld 31,-8(1)
blr
.L17:
lq 10,0(6)
or. 9,10,11
addc 3,11,4
mr 7,11
adde 11,10,5
bne 0,.L12
li 3,0
b .L2
.L16:
li 3,0
blr
Closer to ideal would be
add_unless_zero:
.Lagain:
lqarx 6,0,3
or. 8,6,7
addc 6,6,4
adde 7,7,5
beq 0,.Lfail
stqcx. 6,0,3
bne 0,.Lagain
li 3,1
blr
.Lfail:
li 3,0
blr
