On Fri, Jun 30, 2023 at 5:13 PM Vineet Gupta <vine...@rivosinc.com> wrote:
>
>
>
> On 6/30/23 16:50, Andrew Waterman wrote:
> > I don't believe this is correct; the subtraction is needed to account
> > for the fact that the low part might be negative, resulting in a
> > borrow from the high part. See the output for your test case below:
> >
> > $ cat test.c
> > #include <stdio.h>
> >
> > int main()
> > {
> > unsigned long result, tmp;
> >
> > asm (
> > "li %1,-252645376\n"
> > "addi %1,%1,240\n"
> > "slli %0,%1,32\n"
> > "add %0,%0,%1"
> > : "=r" (result), "=r" (tmp));
> >
> > printf("%lx\n", result);
> >
> > return 0;
> > }
> > $ riscv64-unknown-elf-gcc -O2 test.c
> > $ spike pk a.out
> > bbl loader
> > f0f0f0eff0f0f0f0
> > $
>
> Thx for the quick feedback Andew. I'm clearly lacking in signed math :-(
> So is it possible to have a better code seq for the testcase at all ?
You're welcome!
When Zba is implemented, then inserting a zext.w would do the trick;
see below. (The generalization is that the zext.w is needed if the
32-bit constant is negative.) When Zba is not implemented, I think
the original sequence is optimal.
li a5, -252645376
addi a5, a5, 240
slli a0, a5, 32
zext.w a5, a5
add a0, a0, a5
>
> -Vineet
>
> >
> >
> > On Fri, Jun 30, 2023 at 4:42 PM Vineet Gupta <vine...@rivosinc.com> wrote:
> >>
> >>
> >> On 6/30/23 16:33, Vineet Gupta wrote:
> >>> Ran into a minor snafu in const splitting code when playing with test
> >>> case from an old PR/23813.
> >>>
> >>> long long f(void) { return 0xF0F0F0F0F0F0F0F0ull; }
> >>>
> >>> This currently generates
> >>>
> >>> li a5,-252645376
> >>> addi a5,a5,241
> >>> li a0,-252645376
> >>> slli a5,a5,32
> >>> addi a0,a0,240
> >>> add a0,a5,a0
> >>> ret
> >>>
> >>> The signed math in hival extraction introduces an additional bit,
> >>> causing loval == hival check to fail.
> >>>
> >>> | riscv_split_integer (val=-1085102592571150096, mode=E_DImode) at
> >>> ../gcc/config/riscv/riscv.cc:702
> >>> | 702 unsigned HOST_WIDE_INT loval = sext_hwi (val, 32);
> >>> | (gdb)n
> >>> | 703 unsigned HOST_WIDE_INT hival = sext_hwi ((val - loval) >> 32, 32);
> >>> | (gdb)
> >> FWIW (and I missed adding this observation to the changelog) I pondered
> >> about using unsigned loval/hival with zext_hwi() but that in certain
> >> cases can cause additional insns
> >>
> >> e.g. constant 0x8000_0000 is codegen to LI 1 +SLLI 31 vs, LI
> >> 0xFFFFFFFF_80000000
> >>
> >>
> >>> | 704 rtx hi = gen_reg_rtx (mode), lo = gen_reg_rtx (mode);
> >>> | (gdb) p/x val
> >>> | $2 = 0xf0f0f0f0f0f0f0f0
> >>> | (gdb) p/x loval
> >>> | $3 = 0xfffffffff0f0f0f0
> >>> | (gdb) p/x hival
> >>> | $4 = 0xfffffffff0f0f0f1
> >>> ^^^
> >>> Fix that by eliding the subtraction in shift.
> >>>
> >>> With patch:
> >>>
> >>> li a5,-252645376
> >>> addi a5,a5,240
> >>> slli a0,a5,32
> >>> add a0,a0,a5
> >>> ret
> >>>
> >>> gcc/ChangeLog:
> >>>
> >>> * config/riscv/riscv.cc (riscv_split_integer): hival computation
> >>> do elide subtraction of loval.
> >>> * (riscv_split_integer_cost): Ditto.
> >>> * (riscv_build_integer): Ditto
> >>>
> >>> Signed-off-by: Vineet Gupta <vine...@rivosinc.com>
> >>> ---
> >>> I wasn't planning to do any more work on large const stuff, but just ran
> >>> into it this
> >>> on a random BZ entry when trying search for redundant constant stuff.
> >>> The test seemed too good to pass :-)
> >>> ---
> >>> gcc/config/riscv/riscv.cc | 6 +++---
> >>> 1 file changed, 3 insertions(+), 3 deletions(-)
> >>>
> >>> diff --git a/gcc/config/riscv/riscv.cc b/gcc/config/riscv/riscv.cc
> >>> index 5ac187c1b1b4..377d3aac794b 100644
> >>> --- a/gcc/config/riscv/riscv.cc
> >>> +++ b/gcc/config/riscv/riscv.cc
> >>> @@ -643,7 +643,7 @@ riscv_build_integer (struct riscv_integer_op *codes,
> >>> HOST_WIDE_INT value,
> >>> && (value > INT32_MAX || value < INT32_MIN))
> >>> {
> >>> unsigned HOST_WIDE_INT loval = sext_hwi (value, 32);
> >>> - unsigned HOST_WIDE_INT hival = sext_hwi ((value - loval) >> 32,
> >>> 32);
> >>> + unsigned HOST_WIDE_INT hival = sext_hwi (value >> 32, 32);
> >>> struct riscv_integer_op alt_codes[RISCV_MAX_INTEGER_OPS];
> >>> struct riscv_integer_op hicode[RISCV_MAX_INTEGER_OPS];
> >>> int hi_cost, lo_cost;
> >>> @@ -674,7 +674,7 @@ riscv_split_integer_cost (HOST_WIDE_INT val)
> >>> {
> >>> int cost;
> >>> unsigned HOST_WIDE_INT loval = sext_hwi (val, 32);
> >>> - unsigned HOST_WIDE_INT hival = sext_hwi ((val - loval) >> 32, 32);
> >>> + unsigned HOST_WIDE_INT hival = sext_hwi (val >> 32, 32);
> >>> struct riscv_integer_op codes[RISCV_MAX_INTEGER_OPS];
> >>>
> >>> cost = 2 + riscv_build_integer (codes, loval, VOIDmode);
> >>> @@ -700,7 +700,7 @@ static rtx
> >>> riscv_split_integer (HOST_WIDE_INT val, machine_mode mode)
> >>> {
> >>> unsigned HOST_WIDE_INT loval = sext_hwi (val, 32);
> >>> - unsigned HOST_WIDE_INT hival = sext_hwi ((val - loval) >> 32, 32);
> >>> + unsigned HOST_WIDE_INT hival = sext_hwi (val >> 32, 32);
> >>> rtx hi = gen_reg_rtx (mode), lo = gen_reg_rtx (mode);
> >>>
> >>> riscv_move_integer (lo, lo, loval, mode);
>
