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Updates to describe issues associated with larger vector sizes

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and proposed solutions.

Signed-off-by: Steven Munroe <sjmunroe@us.ibm.com>
pull/30/head
sjmunroe 8 years ago
parent 94f19d4637
commit a9b5e260cf
8 changed files with 191 additions and 20 deletions
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13
Porting_Vector_Intrinsics/bk_main.xml
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@ -39,7 +39,7 @@
<holder>OpenPOWER Foundation</holder>
</copyright>
<!-- TODO: Set the correct document releaseinfo -->
<releaseinfo>Revision 0.2</releaseinfo>
<releaseinfo>Revision 0.3</releaseinfo>
<productname>OpenPOWER</productname>
<pubdate/>

@ -68,6 +68,17 @@

<revhistory>
<!-- TODO: Update as new revisions created -->
<revision>
<date>2017-10-30</date>
<revdescription>
<itemizedlist spacing="compact">
<listitem>
<para>Revision 0.3 - Updates to describe issues associated with larger vector sizes
and proposed solutions.</para>
</listitem>
</itemizedlist>
</revdescription>
</revision>
<revision>
<date>2017-09-14</date>
<revdescription>

11
Porting_Vector_Intrinsics/ch_howto_start.xml
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@ -72,13 +72,22 @@
typedef float __m128 __attribute__ ((__vector_size__ (16), __may_alias__));

/* Internal data types for implementing the intrinsics. */
typedef float __v4sf __attribute__ ((__vector_size__ (16)));
typedef __vector float __v4sf;
/* more typedefs. */

/* The intrinsic implmentations go here. */

#endif /* EMMINTRIN_H_ */]]></programlisting></para>

<note><para>
The interface typedef (__m128) uses the GCC vector builtin
extension syntax, while the internal typedef (__v4sf) uses the altivec
vector extension syntax.
This allows the internal typedefs to work correctly with the PowerPC
overloaded vector builtins. Also we use the __vector (vs vector) type prefix
to avoid name space conflicts with C++.
</para></note>

<para>Then you can start adding small groups of related intrinsic
implementations to the header to be compiled and  examine the generated code.
Once you have what looks like reasonable code you can grep through

81
Porting_Vector_Intrinsics/sec_handling_avx.xml
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@ -75,14 +75,87 @@
the function prologue). This frees up to 64 vectors (32 x 256-bit or 16 x
512-bit structs) for code optimization. </para>

<para>Based on the specifics of our ISA and ABI we will not not use
<para>Based on the specifics of our ISA and ABI we will not use
<literal>__vector_size__</literal> (32) or (64) in the PowerPC implementation of
<literal>__m256</literal> and <literal>__m512</literal>
types. Instead we will typedef structs of 2 or 4 vector (<literal>__m128</literal>) fields. This
types. Instead we will typedef structs of 2 or 4 vector (<literal>__vector</literal>) fields. This
allows efficient handling of these larger data types without requiring new GCC
language extensions. </para>
language extensions or vector builtins. For example:
<programlisting><![CDATA[/* Internal data types for implementing the AVX in PowerISA intrinsics. */
typedef struct __v4df
{
__vector double vd0;
__vector double vd1;
} __vx4df;

<para>In the end we should use the same type names and definitions as the
/* The Intel API is flexible enough that we must allow aliasing with other
vector types, and their scalar components. */
typedef struct __m256d
{
__vector double vd0;
__vector double vd1;
}__attribute__ ((__may_alias__)) __m256d;]]></programlisting></para>

<para>This requires a different syntax for operations
where the 128-bit vector chunks are explicitly referenced.
For example:
<programlisting><![CDATA[extern __inline __mx256d __attribute__((__gnu_inline__, __always_inline__, __artificial__))
_mm256_add_pd (__m256d __A, __m256d __B)
{
__m256d temp;
temp.vd0 = __A.vd0 + __B.vd0;
temp.vd1 = __A.vd1 + __B.vd1;
return (temp);
}]]></programlisting></para>

<para>But this creates a new issue because
the C language does not allow direct casts between structs.
This can be an issue where the intrinsic interface type is not the correct type for the operation.
For example AVX2 integer operations:
<programlisting><![CDATA[
/* The Intel API is flexible enough that we must allow aliasing with other
vector types, and their scalar components. */
typedef struct __m256i
{
__vector long long vdi0;
__vector long long vdi1;
} __m256i;

/* Internal data types for implementing the AVX in PowerISA intrinsics. */
typedef struct __v16hi
{
__vector short vhi0;
__vector short vhi1;
} __v16hi;
]]></programlisting></para>

<para>For the AVX2 intrinsic <literal>_mm256_add_epi16</literal>
we need to cast the input vectors
of 64-bit long long (<literal>__m256i</literal>) into vectors of 16-bit short
(<literal>__v16hi</literal>) before the overloaded add operations.
Here we need to use a pointer reference cast.
For example:
<programlisting><![CDATA[
extern __inline __m256i __attribute__((__gnu_inline__, __always_inline__, __artificial__))
_mx256_add_epi16 (__m256i __A, __m256i __B)
{
__m256i result;
__v16hi a = *((__v16hi *)&__A);
__v16hi b = *((__v16hi *)&__B);
__v16hi c;

c.vhi0 = a.vhi0 + b.vhi0;
c.vhi1 = a.vhi1 + b.vhi1;

result = *((__m256i *)&c);
return (result);
}]]></programlisting></para>
<para>As this and related examples are inlined,
we expect the compiler to recognize this
is a "nop cast" and avoid generating any additional instructions.</para>

<para>In the end we should try
to use the same type names and definitions as the
GCC X86 intrinsic headers where possible. Where that is not possible we can
define new typedefs that provide the best mapping to the underlying PowerISA
hardware.</para>

5
Porting_Vector_Intrinsics/sec_handling_mmx.xml
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@ -35,7 +35,10 @@
implies that MMX integer types can be handled as an internal union of arrays for
the supported element types. So a 64-bit unsigned long long is the best type
for parameter passing and return values, especially for the 64-bit (_si64)
operations as these normally generate a single PowerISA instruction.</para>
operations as these normally generate a single PowerISA instruction.
<phrase revisionflag="added">So for the PowerPC implementation we will define
<literal>__m64</literal> as:</phrase>
<programlisting><![CDATA[typedef __attribute__ ((__aligned__ (8))) unsigned long long __m64;]]></programlisting></para>

<para>The SSE extensions include some copy / convert operations for
<literal>_m128</literal> to /

5
Porting_Vector_Intrinsics/sec_intel_intrinsic_functions.xml
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@ -85,8 +85,9 @@ v2di __builtin_ia32_paddq (v2di, v2di)]]></programlisting></para>

<note><para>A key difference between GCC built-ins for i386 and PowerPC is
that the x86 built-ins have different names of each operation and type while the
PowerPC altivec built-ins tend to have a single generic built-in for  each
operation, across a set of compatible operand types. </para></note>
PowerPC altivec built-ins tend to have a single overloaded
built-in for each operation,
across a set of compatible operand types. </para></note>

<para>In GCC the Intel Intrinsic header (*intrin.h) files are implemented
as a set of inline functions using the Intel Intrinsic API names and types.

87
Porting_Vector_Intrinsics/sec_intel_intrinsic_types.xml
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@ -45,9 +45,17 @@ typedef float __v4sf __attribute__ ((__vector_size__ (16)));]]></programlisting>
<para>There are a couple of issues here:
<itemizedlist spacing="compact">
<listitem>
<para>The API seems to force the compiler to assume
<para>The use of __may_alias__ in the API seems to force the compiler to assume
aliasing of any parameter passed by reference.</para>
</listitem>
<listitem>
<para>
The GCC vector builtin type system (example above) is slightly different
syntax from the original Altivec __vector types. Internally the two typedef forms
may represent the same 128-bit vector type,
but for early source parsing and overloaded vector builtins they are
handled differently.</para>
</listitem>
<listitem>
<para>The data type used at the interface may not be
the correct type for the implied operation.</para>
@ -60,13 +68,16 @@ typedef float __v4sf __attribute__ ((__vector_size__ (16)));]]></programlisting>
(defined as vector long long). </para>

<para>This may not matter when using x86 built-ins but does matter when
the implementation uses C vector extensions or in our case uses PowerPC generic
the implementation uses C vector extensions or in our case using PowerPC
overloaded
vector built-ins
(<xref linkend="sec_powerisa_vector_intrinsics"/>).
For the latter cases the type must be correct for
the compiler to generate the correct type (char, short, int, long)
(<xref linkend="sec_api_implemented"/>) for the generic
builtin operation. There is also concern that excessive use of
the compiler to generate the correct code for the
type (char, short, int, long)
(<xref linkend="sec_api_implemented"/>) for
overloaded builtin operations.
There is also concern that excessive use of
<literal>__may_alias__</literal>
will limit compiler optimization. We are not sure how important this attribute
is to the correct operation of the API.  So at a later stage we should
@ -83,16 +94,76 @@ typedef float __v4sf __attribute__ ((__vector_size__ (16)));]]></programlisting>
implemented as vector attribute extensions of the appropriate  size (  
<literal>__vector_size__</literal> ({8 | 16 | 32, and 64}). For the PowerPC target  GCC currently
only supports the native <literal>__vector_size__</literal> ( 16 ). These we can support directly
in VMX/VSX registers and associated instructions. GCC will compile code with
in VMX/VSX registers and associated instructions.</para>

<para>GCC will compile code with
other   <literal>__vector_size__</literal> values, but the resulting types are treated as simple
arrays of the element type. This does not allow the compiler to use the vector
registers and vector instructions for these (nonnative) vectors.</para>
registers for parameter passing and return values.
For example this intrinsic from immintrin.h:
<programlisting><![CDATA[typedef double __m256d __attribute__ ((__vector_size__ (32), __may_alias__));

extern __inline __m256d __attribute__((__gnu_inline__, __always_inline__, __artificial__))
_mm256_add_pd (__m256d __A, __m256d __B)
{
return (__m256d) ((__v4df)__A + (__v4df)__B);
}
]]></programlisting></para>
<para>And test case:
<programlisting><![CDATA[__m256d
test_mm256_add_pd (__m256d __A, __m256d __B)
{
return (_mm256_add_pd (__A, __B));
}
]]></programlisting></para>
<para>Current GCC generates:
<programlisting><![CDATA[0000000000000970 <test_mm256_add_pd>:
970: 10 00 20 39 li r9,16
974: 98 26 80 7d lxvd2x vs12,0,r4
978: 98 2e 40 7d lxvd2x vs10,0,r5
97c: 20 00 e0 38 li r7,32
980: f8 ff e1 fb std r31,-8(r1)
984: b1 ff 21 f8 stdu r1,-80(r1)
988: 30 00 00 39 li r8,48
98c: 98 4e 04 7c lxvd2x vs0,r4,r9
990: 98 4e 65 7d lxvd2x vs11,r5,r9
994: 00 53 8c f1 xvadddp vs12,vs12,vs10
998: 00 00 c1 e8 ld r6,0(r1)
99c: 78 0b 3f 7c mr r31,r1
9a0: 00 5b 00 f0 xvadddp vs0,vs0,vs11
9a4: c1 ff c1 f8 stdu r6,-64(r1)
9a8: 98 3f 9f 7d stxvd2x vs12,r31,r7
9ac: 98 47 1f 7c stxvd2x vs0,r31,r8
9b0: 98 3e 9f 7d lxvd2x vs12,r31,r7
9b4: 98 46 1f 7c lxvd2x vs0,r31,r8
9b8: 50 00 3f 38 addi r1,r31,80
9bc: f8 ff e1 eb ld r31,-8(r1)
9c0: 98 1f 80 7d stxvd2x vs12,0,r3
9c4: 98 4f 03 7c stxvd2x vs0,r3,r9
9c8: 20 00 80 4e blr]]></programlisting></para>

<para>The compiler treats the parameters and return value
as scalar arrays, which are passed by reference.
The operation is vectorized in this case,
but the 256-bit result is returned through storage.</para>

<para>This is not what we want to see for a simple 4 by double add.
It would be better if we can pass and return
MMX (<xref linkend="sec_handling_mmx"/>) and AVX (<xref linkend="sec_handling_avx"/>)
values as PowerPC registers and avoid the storage references.
If we can get the parameter and return values as registers,
this example will reduce to:
<programlisting><![CDATA[0000000000000970 <test_mx256_add_pd>:
970: xvadddp vs34,vs34,vs36
974: xvadddp vs35,vs35,vs37
978: blr]]></programlisting></para>

<para>So the PowerISA VMX/VSX facilities and GCC compiler support for
128-bit/16-byte vectors and associated vector built-ins
are well matched to implementing equivalent X86 SSE intrinsic functions.
However implementing the older MMX (64-bit) and the latest
AVX (256 / 512-bit) extensions requires more thought and some ingenuity.</para>
AVX (256 / 512-bit) extensions requires more thought and some
ingenuity.</para>
<xi:include href="sec_handling_mmx.xml"/>
<xi:include href="sec_handling_avx.xml"/>

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Porting_Vector_Intrinsics/sec_powerisa_vector_intrinsics.xml
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@ -37,8 +37,9 @@
Vector Built-in Functions</emphasis></emphasis>.</para>

<para>Appendix A is organized (sorted) by built-in name, output type, then
parameter types. Most built-ins are generic as the named operation (add,
sub, mul, cmpeq, ...) applies to multiple types. </para>
parameter types. Most built-ins are overloaded
as the named operation (vec_add,
vec_sub, vec_mul, vec_cmpeq, ...) applies to multiple types. </para>

<para>So the <literal>vec_add</literal> built-in applies to all the signed and unsigned
integer types (char, short, in, and long) plus float and double floating-point
@ -59,7 +60,8 @@ vector float vec_add (vector float, vector float);
vector double vec_add (vector double, vector double);]]></programlisting></para>

<para>This is one key difference between PowerISA built-ins and Intel
Intrinsics (Intel Intrinsics are not generic and include type information in
Intrinsics (Intel Intrinsics are not overloaded
and include type information in
the name). This is why it is so important to understand the vector element
types and to add the appropriate type casts to get the correct results.</para>


1
Porting_Vector_Intrinsics/sec_simple_examples.xml
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@ -48,6 +48,7 @@ _mm_mullo_epi16 (__m128i __A, __m128i __B)
<para>Note this requires a cast for the compiler to generate the correct
code for the intended operation. The parameters and result are the generic
interface
type <literal>__m128i</literal>, which is a vector long long with the
<literal>__may_alias__</literal> attribute. But
operation is a vector multiply low on unsigned short elements.

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