https://gcc.gnu.org/bugzilla/show_bug.cgi?id=91635
--- Comment #19 from Jim Wilson <wilson at gcc dot gnu.org> ---
I did cross toolchain builds and checks for rv32i/newlib and rv64gc/glibc.
There were no regressions.
Since the splitters exist to reduce code size, I looked at the libc and
libstdc++ sizes with and without the patch.
For the rv32i build, libstdc++ is the same size, and libc is 4 bytes smaller.
I extracted a testcase.
int
isnanf (float x)
{
union u { float f; int i; } u;
u.f = x;
int i = u.i;
i &= 0x7fffffff;
return i > 0x7f800000;
}
Without the patch with -O2 -S we get
slli a0,a0,1
li a5,2139095040
srli a0,a0,1
sgt a0,a0,a5
ret
with the patch we get
li a5,-16777216
slli a0,a0,1
sgtu a0,a0,a5
ret
The issue here is that the extra temporary used for the splitter that generates
the slli/srli gives combine a little more freedom, and it manages to find an
optimization that was missed before.
For the rv64gc build, libc is 38 bytes smaller and libstdc++ is 34 bytes
larger. The libc issue appears to be the same as above. The libstdc++ issue
is more interesting. I extracted a testcase.
unsigned long
sub (long l)
{
union u {
struct s { int a : 19; unsigned int b : 13; int x; } s;
long l;
} u;
u.l = l;
return u.s.b;
}
Without the patch, with -O2 -S, we generate
srliw a0,a0,19
ret
and with the patch we get
srli a0,a0,19
slli a0,a0,51
srli a0,a0,51
ret
The issue here is that there is no REG_DEAD note for the temporary that we
generated. So without the patch combine generates
Successfully matched this instruction:
(set (reg:DI 78)
(zero_extract:DI (reg:DI 82)
(const_int 13 [0xd])
(const_int 19 [0x13])))
and with the patch combine generates
Failed to match this instruction:
(parallel [
(set (reg:DI 78)
(zero_extract:DI (reg:DI 82)
(const_int 13 [0xd])
(const_int 19 [0x13])))
(set (reg:DI 83)
(and:DI (ashift:DI (reg:DI 82)
(const_int 32 [0x20]))
(const_int -2251799813685248 [0xfff8000000000000])))
])
where reg 83 is the temporary generated by the splitter which has no REG_DEAD
note.
Since the temporary appears to be generally useful, and lack of a REG_DEAD note
causes problems, I think it would be better to modify the two define_split
patterns to have a clobber instead of calling gen_reg_rtx. I do get the
REG_DEAD note in this case, and the correct single instruction output for the
second testcase. The three define_insn_and_split patterns can remain the same
as they aren't causing any trouble.
Though I suppose there is a question here of whether combine will ever
accidentally substitute in a paradoxical reg for a clobber. I would hope that
it doesn't. I can check for that.
Since I have a new patch, and new testcases, I need to redo the builds and
checks from scratch.
