optabs: Make all `*dot_prod_optab's modeled as conversions

Given the specification in the GCC internals manual defines the
{u|s}dot_prod<m> standard name as taking "two signed elements of the
same mode, adding them to a third operand of wider mode", there is
currently ambiguity in the relationship between the mode of the first
two arguments and that of the third.

This vagueness means that, in theory, different modes may be
supportable in the third argument.  This flexibility would allow for a
given backend to add to the accumulator a different number of
vectorized products, e.g. A backend may provide instructions for both:

  accum += a[0] * b[0] + a[1] * b[1] + a[2] * b[2] + a[3] * b[3]

and

  accum += a[0] * b[0] + a[1] * b[1],

as is now seen in the SVE2.1 extension to AArch64.  In spite of the
aforementioned flexibility, modeling the dot-product operation as a
direct optab means that we have no way to encode both input and the
accumulator data modes into the backend pattern name, which prevents
us from harnessing this flexibility.

We therefore make all dot_prod optabs conversions, allowing, for
example, for the encoding of both 2-way and 4-way dot product backend
patterns.

gcc/ChangeLog:

	* optabs.def (sdot_prod_optab): Convert from OPTAB_D to
	OPTAB_CD.
	(udot_prod_optab): Likewise.
	(usdot_prod_optab): Likewise.
	* doc/md.texi (Standard Names): update entries for u,s and us
	dot_prod names.

2 files changed, 25 insertions, 27 deletions

diff --git a/gcc/doc/md.texi b/gcc/doc/md.texi
index a925911..7001daf 100644
--- a/gcc/doc/md.texi
+++ b/gcc/doc/md.texi
@@ -5760,15 +5760,14 @@ for (i = 0; i < LEN + BIAS; i++)
     operand0 += operand2[i];
 @end smallexample
 
-@cindex @code{sdot_prod@var{m}} instruction pattern
-@item @samp{sdot_prod@var{m}}
-
-Compute the sum of the products of two signed elements.
-Operand 1 and operand 2 are of the same mode. Their
-product, which is of a wider mode, is computed and added to operand 3.
-Operand 3 is of a mode equal or wider than the mode of the product. The
-result is placed in operand 0, which is of the same mode as operand 3.
-@var{m} is the mode of operand 1 and operand 2.
+@cindex @code{sdot_prod@var{m}@var{n}} instruction pattern
+@item @samp{sdot_prod@var{m}@var{n}}
+
+Multiply operand 1 by operand 2 without loss of precision, given that
+both operands contain signed elements.  Add each product to the overlapping
+element of operand 3 and store the result in operand 0.  Operands 0 and 3
+have mode @var{m} and operands 1 and 2 have mode @var{n}, with @var{n}
+having narrower elements than @var{m}.
 
 Semantically the expressions perform the multiplication in the following signs
 
@@ -5778,15 +5777,14 @@ sdot<signed op0, signed op1, signed op2, signed op3> ==
 @dots{}
 @end smallexample
 
-@cindex @code{udot_prod@var{m}} instruction pattern
-@item @samp{udot_prod@var{m}}
+@cindex @code{udot_prod@var{m}@var{n}} instruction pattern
+@item @samp{udot_prod@var{m}@var{n}}
 
-Compute the sum of the products of two unsigned elements.
-Operand 1 and operand 2 are of the same mode. Their
-product, which is of a wider mode, is computed and added to operand 3.
-Operand 3 is of a mode equal or wider than the mode of the product. The
-result is placed in operand 0, which is of the same mode as operand 3.
-@var{m} is the mode of operand 1 and operand 2.
+Multiply operand 1 by operand 2 without loss of precision, given that
+both operands contain unsigned elements.  Add each product to the overlapping
+element of operand 3 and store the result in operand 0.  Operands 0 and 3
+have mode @var{m} and operands 1 and 2 have mode @var{n}, with @var{n}
+having narrower elements than @var{m}.
 
 Semantically the expressions perform the multiplication in the following signs
 
@@ -5796,14 +5794,14 @@ udot<unsigned op0, unsigned op1, unsigned op2, unsigned op3> ==
 @dots{}
 @end smallexample
 
-@cindex @code{usdot_prod@var{m}} instruction pattern
-@item @samp{usdot_prod@var{m}}
+@cindex @code{usdot_prod@var{m}@var{n}} instruction pattern
+@item @samp{usdot_prod@var{m}@var{n}}
 Compute the sum of the products of elements of different signs.
-Operand 1 must be unsigned and operand 2 signed. Their
-product, which is of a wider mode, is computed and added to operand 3.
-Operand 3 is of a mode equal or wider than the mode of the product. The
-result is placed in operand 0, which is of the same mode as operand 3.
-@var{m} is the mode of operand 1 and operand 2.
+Multiply operand 1 by operand 2 without loss of precision, given that operand 1
+is unsigned and operand 2 is signed.  Add each product to the overlapping
+element of operand 3 and store the result in operand 0.  Operands 0 and 3 have
+mode @var{m} and operands 1 and 2 have mode @var{n}, with @var{n} having
+narrower elements than @var{m}.
 
 Semantically the expressions perform the multiplication in the following signs
 
diff --git a/gcc/optabs.def b/gcc/optabs.def
index 58a9394..ba86014 100644
--- a/gcc/optabs.def
+++ b/gcc/optabs.def
@@ -110,6 +110,9 @@ OPTAB_CD(mask_scatter_store_optab, "mask_scatter_store$a$b")
 OPTAB_CD(mask_len_scatter_store_optab, "mask_len_scatter_store$a$b")
 OPTAB_CD(vec_extract_optab, "vec_extract$a$b")
 OPTAB_CD(vec_init_optab, "vec_init$a$b")
+OPTAB_CD (sdot_prod_optab, "sdot_prod$I$a$b")
+OPTAB_CD (udot_prod_optab, "udot_prod$I$a$b")
+OPTAB_CD (usdot_prod_optab, "usdot_prod$I$a$b")
 
 OPTAB_CD (while_ult_optab, "while_ult$a$b")
 
@@ -413,10 +416,7 @@ OPTAB_D (savg_floor_optab, "avg$a3_floor")
 OPTAB_D (uavg_floor_optab, "uavg$a3_floor")
 OPTAB_D (savg_ceil_optab, "avg$a3_ceil")
 OPTAB_D (uavg_ceil_optab, "uavg$a3_ceil")
-OPTAB_D (sdot_prod_optab, "sdot_prod$I$a")
 OPTAB_D (ssum_widen_optab, "widen_ssum$I$a3")
-OPTAB_D (udot_prod_optab, "udot_prod$I$a")
-OPTAB_D (usdot_prod_optab, "usdot_prod$I$a")
 OPTAB_D (usum_widen_optab, "widen_usum$I$a3")
 OPTAB_D (usad_optab, "usad$I$a")
 OPTAB_D (ssad_optab, "ssad$I$a")