arm: Avoid using .dn register aliases

clang now (in the upcoming 5.0 version) is capable of building our
arm assembly without relying on gas-preprocessor, although clang/LLVM
doesn't support .dn register aliases.

The VC1 MC assembly was only built and used if the chosen assembler
supported the .dn directives though. This was supported as long as
gas-preprocessor was used.

This means that VC1 decoding got a speed regression on clang 5.0,
unless the user manually chose using gas-preprocessor again.

By avoiding using the .dn register aliases, we can build the VC1 MC
assembly with the latest clang version.

Support for the .dn/.qn directives in clang/LLVM isn't actively planned,
see https://bugs.llvm.org/show_bug.cgi?id=18199.

This partially reverts 896a5bff64.

Signed-off-by: Martin Storsjö <martin@martin.st>

configure

@@ -1662,7 +1662,6 @@ SYSTEM_FUNCS="
 
 TOOLCHAIN_FEATURES="
     as_arch_directive
-    as_dn_directive
     as_fpu_directive
     as_func
     as_object_arch
@@ -4379,10 +4378,6 @@ EOF
 
     check_as <<EOF && enable as_arch_directive
 .arch armv7-a
-EOF
-    check_as <<EOF && enable as_dn_directive
-ra .dn d0.i16
-.unreq ra
 EOF
     check_as <<EOF && enable as_fpu_directive
 .fpu neon

libavcodec/arm/vc1dsp_init_neon.c

@@ -22,8 +22,6 @@
 #include "libavcodec/vc1dsp.h"
 #include "vc1dsp.h"
 
-#include "config.h"
-
 void ff_vc1_inv_trans_8x8_neon(int16_t *block);
 void ff_vc1_inv_trans_4x8_neon(uint8_t *dest, ptrdiff_t stride, int16_t *block);
 void ff_vc1_inv_trans_8x4_neon(uint8_t *dest, ptrdiff_t stride, int16_t *block);
@@ -93,7 +91,6 @@ av_cold void ff_vc1dsp_init_neon(VC1DSPContext *dsp)
     dsp->vc1_inv_trans_4x4_dc = ff_vc1_inv_trans_4x4_dc_neon;
 
     dsp->put_vc1_mspel_pixels_tab[ 0] = ff_put_pixels8x8_neon;
-    if (HAVE_AS_DN_DIRECTIVE) {
     dsp->put_vc1_mspel_pixels_tab[ 1] = ff_put_vc1_mspel_mc10_neon;
     dsp->put_vc1_mspel_pixels_tab[ 2] = ff_put_vc1_mspel_mc20_neon;
     dsp->put_vc1_mspel_pixels_tab[ 3] = ff_put_vc1_mspel_mc30_neon;
@@ -109,7 +106,6 @@ av_cold void ff_vc1dsp_init_neon(VC1DSPContext *dsp)
     dsp->put_vc1_mspel_pixels_tab[13] = ff_put_vc1_mspel_mc13_neon;
     dsp->put_vc1_mspel_pixels_tab[14] = ff_put_vc1_mspel_mc23_neon;
     dsp->put_vc1_mspel_pixels_tab[15] = ff_put_vc1_mspel_mc33_neon;
-    }
 
     dsp->put_no_rnd_vc1_chroma_pixels_tab[0] = ff_put_vc1_chroma_mc8_neon;
     dsp->avg_no_rnd_vc1_chroma_pixels_tab[0] = ff_avg_vc1_chroma_mc8_neon;

libavcodec/arm/vc1dsp_neon.S

@@ -663,7 +663,6 @@ function ff_vc1_inv_trans_4x4_neon, export=1
         bx              lr
 endfunc
 
-#if HAVE_AS_DN_DIRECTIVE
 @ The absolute value of multiplication constants from vc1_mspel_filter and vc1_mspel_{ver,hor}_filter_16bits.
 @ The sign is embedded in the code below that carries out the multiplication (mspel_filter{,.16}).
 #define MSPEL_MODE_1_MUL_CONSTANTS  4 53 18 3
@@ -689,22 +688,18 @@ endfunc
 
 @ Setup constants in registers for a subsequent use of mspel_filter{,.16}.
 .macro mspel_constants typesize reg_a reg_b reg_c reg_d filter_a filter_b filter_c filter_d reg_add filter_add_register
-  @ Define double-word register aliases. Typesize should be i8 or i16.
+  @ Typesize should be i8 or i16.
-  ra .dn \reg_a\().\typesize
-  rb .dn \reg_b\().\typesize
-  rc .dn \reg_c\().\typesize
-  rd .dn \reg_d\().\typesize
 
   @ Only set the register if the value is not 1 and unique
   .if \filter_a != 1
-        vmov            ra,  #\filter_a              @ ra = filter_a
+        vmov.\typesize  \reg_a,  #\filter_a          @ reg_a = filter_a
   .endif
-        vmov            rb,  #\filter_b              @ rb = filter_b
+        vmov.\typesize  \reg_b,  #\filter_b          @ reg_b = filter_b
   .if \filter_b != \filter_c
-        vmov            rc,  #\filter_c              @ rc = filter_c
+        vmov.\typesize  \reg_c,  #\filter_c          @ reg_c = filter_c
   .endif
   .if \filter_d != 1
-        vmov            rd,  #\filter_d              @ rd = filter_d
+        vmov.\typesize  \reg_d,  #\filter_d          @ reg_d = filter_d
   .endif
   @ vdup to double the size of typesize
   .ifc \typesize,i8
@@ -712,11 +707,6 @@ endfunc
   .else
         vdup.32         \reg_add,  \filter_add_register     @ reg_add = filter_add_register
   .endif
-
-  .unreq ra
-  .unreq rb
-  .unreq rc
-  .unreq rd
 .endm
 
 @ After mspel_constants has been used, do the filtering.
@@ -987,7 +977,6 @@ PUT_VC1_MSPEL_MC_V_ONLY(2)
 PUT_VC1_MSPEL_MC_V_ONLY(3)
 
 #undef PUT_VC1_MSPEL_MC_V_ONLY
-#endif
 
 function ff_put_pixels8x8_neon, export=1
         vld1.64         {d0}, [r1], r2