On Tue, 25 Apr 2023 at 16:29, Richard Sandiford
<richard.sandif...@arm.com> wrote:
>
> Prathamesh Kulkarni <prathamesh.kulka...@linaro.org> writes:
> > Hi Richard,
> > While digging thru aarch64_expand_vector_init, I noticed it gives
> > priority to loading a constant first:
> > /* Initialise a vector which is part-variable. We want to first try
> > to build those lanes which are constant in the most efficient way we
> > can. */
> >
> > which results in suboptimal code-gen for following case:
> > int16x8_t f_s16(int16_t x)
> > {
> > return (int16x8_t) { x, x, x, x, x, x, x, 1 };
> > }
> >
> > code-gen trunk:
> > f_s16:
> > movi v0.8h, 0x1
> > ins v0.h[0], w0
> > ins v0.h[1], w0
> > ins v0.h[2], w0
> > ins v0.h[3], w0
> > ins v0.h[4], w0
> > ins v0.h[5], w0
> > ins v0.h[6], w0
> > ret
> >
> > The attached patch tweaks the following condition:
> > if (n_var == n_elts && n_elts <= 16)
> > {
> > ...
> > }
> >
> > to pass if maxv >= 80% of n_elts, with 80% being an
> > arbitrary "high enough" threshold. The intent is to dup
> > the most repeating variable if it it's repetition
> > is "high enough" and insert constants which should be "better" than
> > loading constant first and inserting variables like in the above case.
>
> I'm not too keen on the 80%. Like you say, it seems a bit arbitrary.
>
> The case above can also be handled by relaxing n_var == n_elts to
> n_var >= n_elts - 1, so that if there's just one constant element,
> we look for duplicated variable elements. If there are none
> (maxv == 1), but there is a constant element, we can duplicate
> the constant element into a register.
>
> The case when there's more than one constant element needs more thought
> (and testcases :-)). E.g. after a certain point, it would probably be
> better to load the variable and constant parts separately and blend them
> using TBL. It also matters whether the constants are equal or not.
>
> There are also cases that could be handled using EXT.
>
> Plus, if we're inserting many variable elements that are already
> in GPRs, we can probably do better by coalescing them into bigger
> GPR values and inserting them as wider elements.
>
> Because of things like that, I think we should stick to the
> single-constant case for now.
Hi Richard,
Thanks for the suggestions. The attached patch only handles the single
constant case.
Bootstrap+test in progress on aarch64-linux-gnu.
Does it look OK ?
Thanks,
Prathamesh
>
> Thanks,
> Richard
[aarch64] Improve code-gen for vector initialization with single constant
element.
gcc/ChangeLog:
* config/aarch64/aarc64.cc (aarch64_expand_vector_init): Tweak condition
if (n_var == n_elts && n_elts <= 16) to allow a single constant,
and if maxv == 1, use constant element for duplicating into register.
gcc/testsuite/ChangeLog:
* gcc.target/aarch64/vec-init-single-const.c: New test.
diff --git a/gcc/config/aarch64/aarch64.cc b/gcc/config/aarch64/aarch64.cc
index 2b0de7ca038..f46750133a6 100644
--- a/gcc/config/aarch64/aarch64.cc
+++ b/gcc/config/aarch64/aarch64.cc
@@ -22167,7 +22167,7 @@ aarch64_expand_vector_init (rtx target, rtx vals)
and matches[X][1] with the count of duplicate elements (if X is the
earliest element which has duplicates). */
- if (n_var == n_elts && n_elts <= 16)
+ if ((n_var >= n_elts - 1) && n_elts <= 16)
{
int matches[16][2] = {0};
for (int i = 0; i < n_elts; i++)
@@ -22227,6 +22227,18 @@ aarch64_expand_vector_init (rtx target, rtx vals)
vector register. For big-endian we want that position to hold
the last element of VALS. */
maxelement = BYTES_BIG_ENDIAN ? n_elts - 1 : 0;
+
+ /* If we have a single constant element, use that for duplicating
+ instead. */
+ if (n_var == n_elts - 1)
+ for (int i = 0; i < n_elts; i++)
+ if (CONST_INT_P (XVECEXP (vals, 0, i))
+ || CONST_DOUBLE_P (XVECEXP (vals, 0, i)))
+ {
+ maxelement = i;
+ break;
+ }
+
rtx x = force_reg (inner_mode, XVECEXP (vals, 0, maxelement));
aarch64_emit_move (target, lowpart_subreg (mode, x, inner_mode));
}
diff --git a/gcc/testsuite/gcc.target/aarch64/vec-init-single-const.c
b/gcc/testsuite/gcc.target/aarch64/vec-init-single-const.c
new file mode 100644
index 00000000000..517f47b13ec
--- /dev/null
+++ b/gcc/testsuite/gcc.target/aarch64/vec-init-single-const.c
@@ -0,0 +1,66 @@
+/* { dg-do compile } */
+/* { dg-options "-O2" } */
+/* { dg-final { check-function-bodies "**" "" "" } } */
+
+#include <arm_neon.h>
+
+/*
+** f_s8:
+** ...
+** dup v[0-9]+\.16b, w[0-9]+
+** movi v[0-9]+\.8b, 0x1
+** ins v[0-9]+\.b\[15\], v[0-9]+\.b\[0\]
+** ...
+** ret
+*/
+
+int8x16_t f_s8(int8_t x)
+{
+ return (int8x16_t) { x, x, x, x, x, x, x, x,
+ x, x, x, x, x, x, x, 1 };
+}
+
+/*
+** f_s16:
+** ...
+** dup v[0-9]+\.8h, w[0-9]+
+** movi v[0-9]+\.4h, 0x1
+** ins v[0-9]+\.h\[7\], v[0-9]+\.h\[0\]
+** ...
+** ret
+*/
+
+int16x8_t f_s16(int16_t x)
+{
+ return (int16x8_t) { x, x, x, x, x, x, x, 1 };
+}
+
+/*
+** f_s32:
+** ...
+** movi v[0-9]\.2s, 0x1
+** dup v[0-9]\.4s, w[0-9]+
+** ins v[0-9]+\.s\[3\], v[0-9]+\.s\[0\]
+** ...
+** ret
+*/
+
+int32x4_t f_s32(int32_t x)
+{
+ return (int32x4_t) { x, x, x, 1 };
+}
+
+/*
+** f_s64:
+** ...
+** fmov d[0-9]+, x[0-9]+
+** mov x[0-9]+, 1
+** ins v[0-9]+\.d\[1\], x[0-9]+
+** ...
+** ret
+*/
+
+int64x2_t f_s64(int64_t x)
+{
+ return (int64x2_t) { x, 1 };
+}
