x86: introduce .hfloat directive

This is to be able to generate data passed to {,V}CVTPH2PS and acted
upon by AVX512-FP16 insns. To be able to also use the hex forms
supported for other floating point formats, a small addition to the
generic hex_float() is needed.

Extend existing x86 testcases.

gas/config/tc-i386.c

@@ -512,7 +512,7 @@ const char EXP_CHARS[] = "eE";
 /* Chars that mean this number is a floating point constant
    As in 0f12.456
    or    0d1.2345e12.  */
-const char FLT_CHARS[] = "fFdDxX";
+const char FLT_CHARS[] = "fFdDxXhH";
 
 /* Tables for lexical analysis.  */
 static char mnemonic_chars[256];
@@ -1356,6 +1356,7 @@ const pseudo_typeS md_pseudo_table[] =
   {"ffloat", float_cons, 'f'},
   {"dfloat", float_cons, 'd'},
   {"tfloat", float_cons, 'x'},
+  {"hfloat", float_cons, 'h'},
   {"value", cons, 2},
   {"slong", signed_cons, 4},
   {"noopt", s_ignore, 0},

gas/doc/c-i386.texi

@@ -1313,18 +1313,21 @@ data type.  Constructors build these data types into memory.
 @cindex @code{single} directive, i386
 @cindex @code{double} directive, i386
 @cindex @code{tfloat} directive, i386
+@cindex @code{hfloat} directive, i386
 @cindex @code{float} directive, x86-64
 @cindex @code{single} directive, x86-64
 @cindex @code{double} directive, x86-64
 @cindex @code{tfloat} directive, x86-64
+@cindex @code{hfloat} directive, x86-64
 @itemize @bullet
 @item
 Floating point constructors are @samp{.float} or @samp{.single},
-@samp{.double}, and @samp{.tfloat} for 32-, 64-, and 80-bit formats.
-These correspond to instruction mnemonic suffixes @samp{s}, @samp{l},
-and @samp{t}. @samp{t} stands for 80-bit (ten byte) real.  The 80387
-only supports this format via the @samp{fldt} (load 80-bit real to stack
-top) and @samp{fstpt} (store 80-bit real and pop stack) instructions.
+@samp{.double}, @samp{.tfloat}, and @samp{.hfloat} for 32-, 64-, 80-, and
+16-bit formats respectively. The former three correspond to instruction
+mnemonic suffixes @samp{s}, @samp{l}, and @samp{t}. @samp{t} stands for
+80-bit (ten byte) real.  The 80387 only supports this format via the
+@samp{fldt} (load 80-bit real to stack top) and @samp{fstpt} (store 80-bit
+real and pop stack) instructions.
 
 @cindex @code{word} directive, i386
 @cindex @code{long} directive, i386

gas/read.c

@@ -4852,6 +4852,11 @@ hex_float (int float_type, char *bytes)
 
   switch (float_type)
     {
+    case 'h':
+    case 'H':
+      length = 2;
+      break;
+
     case 'f':
     case 'F':
     case 's':

gas/testsuite/gas/i386/fp-elf32.d

@@ -14,3 +14,4 @@ Contents of section .data:
  0060 00000000 00000080 fe3f0000 00000000  .*
  0070 00000080 fdbf0000 00000000 00000080  .*
  0080 ff030000 aaaaaaaa aaaaaaaa aaaaaaaa  .*
+ 0090 003c00c0 003c5555 55555555 55555555  .*

gas/testsuite/gas/i386/fp-elf64.d

@@ -14,3 +14,4 @@ Contents of section .data:
  0060 00000000 00000080 fe3f0000 00000000  .*
  0070 00000000 00000080 fdbf0000 00000000  .*
  0080 00000000 00000080 ff030000 00000000  .*
+ 0090 003c00c0 003c5555 55555555 55555555  .*

gas/testsuite/gas/i386/fp.d

@@ -12,3 +12,4 @@ Contents of section .data:
  0050 ffffffff ffffffff ffffcccc cccccccc  .*
  0060 00000000 00000080 fe3f0000 00000000  .*
  0070 0080fdbf 00000000 00000080 ff03aaaa  .*
+ 0080 003c00c0 003c5555 55555555 55555555  .*

gas/testsuite/gas/i386/fp.s

@@ -29,3 +29,6 @@
 	.dc.x 0x:bffd80
 	.dcb.x 1, 0x:03ff80
 	.p2align 4,0xaa
+
+	.hfloat 1, -2, 0x:3c00
+	.p2align 4,0x55