https://gcc.gnu.org/bugzilla/show_bug.cgi?id=68908
Bug ID: 68908
Summary: inefficient code for an atomic preincrement on
powerpc64le
Product: gcc
Version: 6.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: target
Assignee: unassigned at gcc dot gnu.org
Reporter: msebor at gcc dot gnu.org
Target Milestone: ---
GCC 6.0 for powerpc64le emits the following inefficient code for an atomic
increment. (The bogus warning is the subject of bug 68907).
$ cat ~/tmp/t.c && /build/gcc-trunk/gcc/xgcc -B /build/gcc-trunk/gcc -O2 -S
-Wall -o/dev/stdout ~/tmp/t.c
typedef _Atomic int AI;
void increment (AI *ai)
{
++*ai;
}
.file "t.c"
.machine power8
.abiversion 2
.section ".toc","aw"
.section ".text"
/home/remote/msebor/tmp/t.c: In function ‘increment’:
/home/remote/msebor/tmp/t.c:5:5: warning: right-hand operand of comma
expression has no effect [-Wunused-value]
++*ai;
^~~~~
.align 2
.p2align 4,,15
.globl increment
.type increment, @function
increment:
sync
lwz 9,0(3)
cmpw 7,9,9
bne- 7,$+4
isync
extsw 9,9
stw 9,-16(1)
.p2align 4,,15
.L6:
lwz 8,-16(1)
addi 9,9,1
sync
.L2:
lwarx 10,0,3
cmpw 0,10,8
bne- 0,.L3
stwcx. 9,0,3
bne- 0,.L2
.L3:
isync
mfcr 9,128
rlwinm 9,9,3,1
beq 0,.L4
stw 10,-16(1)
.L4:
andi. 10,9,0x1
bnelr- 0
lwa 9,-16(1)
b .L6
.long 0
.byte 0,0,0,0,0,0,0,0
.size increment,.-increment
.ident "GCC: (GNU) 6.0.0 20151214 (experimental)"
.section .note.GNU-stack,"",@progbits
Clang 3.8 emits the expected:
increment: # @increment
.Lfunc_begin0:
# BB#0: # %entry
li 4, 1
sync
.LBB0_1: # %entry
# =>This Inner Loop Header: Depth=1
lwarx 5, 0, 3
add 5, 4, 5
stwcx. 5, 0, 3
bne 0, .LBB0_1
# BB#2: # %entry
lwsync
blr
