https://gcc.gnu.org/bugzilla/show_bug.cgi?id=100694
Bug ID: 100694
Summary: PPC: initialization of __int128 is very inefficient
Product: gcc
Version: 8.3.1
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: target
Assignee: unassigned at gcc dot gnu.org
Reporter: jens.seifert at de dot ibm.com
Target Milestone: ---
Initializing a __int128 from 2 64-bit integers is implemented very inefficient.
The most natural code which works good on all other platforms generate
additional 2 li 0 + 2 or instructions.
void test2(unsigned __int128* res, unsigned long long hi, unsigned long long
lo)
{
unsigned __int128 i = hi;
i <<= 64;
i |= lo;
*res = i;
}
_Z5test2Poyy:
.LFB15:
.cfi_startproc
li 8,0
li 11,0
or 10,5,8
or 11,11,4
std 10,0(3)
std 11,8(3)
blr
.long 0
.byte 0,9,0,0,0,0,0,0
.cfi_endproc
While for the above sample, "+" instead "|" solves the issues, it generates
addc+addz in other more complicated scenarsion.
The most ugly workaround I can think of I now use as workaround.
void test4(unsigned __int128* res, unsigned long long hi, unsigned long long
lo)
{
union
{ unsigned __int128 i;
struct
{
unsigned long long lo;
unsigned long long hi;
} s;
} u;
u.s.lo = lo;
u.s.hi = hi;
*res = u.i;
}
This generates the expected code sequence in all cases I have looked at.
_Z5test4Poyy:
.LFB17:
.cfi_startproc
std 5,0(3)
std 4,8(3)
blr
.long 0
.byte 0,9,0,0,0,0,0,0
.cfi_endproc
Please merge li 0 + or to nop.
