[PATCH] gdb: Add support for fp8 formats

[<Ezra.Sitorus@arm.com>]

From: Ezra Sitorus <ezra.sitorus@arm.com>

FP8 introduces 2 new formats: E5M2 and E4M3. E5M2 follows IEEE
conventions for exponents and special values. On the other hand, E4M3
increases the dynamic range by having fewer special values - there is no
infinity, and only one encoding for NaN. This has been accounted for in
gdb/target-float.c: floatformat_classify and from_target.

One particular thing I would like to highlight is that since e4m3 has
only 1 NaN encoding - all ones - this will be printed as nan(0x7). You
can see this in the test cases later. I would like to ask if there is a
better way of handling this?

More information about FP8 can be found in the technical paper here:
https://arxiv.org/abs/2209.05433
---
 gdb/gdbtypes.c            |  8 ++++++++
 gdb/gdbtypes.h            |  2 ++
 gdb/target-descriptions.c | 10 ++++++++++
 gdb/target-float.c        | 12 +++++++++++-
 gdbsupport/tdesc.cc       |  4 +++-
 gdbsupport/tdesc.h        |  2 ++
 include/floatformat.h     |  5 +++++
 7 files changed, 41 insertions(+), 2 deletions(-)

diff --git a/gdb/gdbtypes.c b/gdb/gdbtypes.c
index 24e6d0bf8f5..f07c259e25b 100644
--- a/gdb/gdbtypes.c
+++ b/gdb/gdbtypes.c
@@ -79,6 +79,14 @@ const struct rank NS_INTEGER_POINTER_CONVERSION_BADNESS = {3,0};
 const struct rank VARARG_BADNESS = {4, 0};
 
 /* Floatformat pairs.  */
+const struct floatformat *floatformats_fp8_e5m2[BFD_ENDIAN_UNKNOWN] = {
+  &floatformat_fp8_e5m2_big,
+  &floatformat_fp8_e5m2_little
+};
+const struct floatformat *floatformats_fp8_e4m3[BFD_ENDIAN_UNKNOWN] = {
+  &floatformat_fp8_e4m3_big,
+  &floatformat_fp8_e4m3_little
+};
 const struct floatformat *floatformats_ieee_half[BFD_ENDIAN_UNKNOWN] = {
   &floatformat_ieee_half_big,
   &floatformat_ieee_half_little
diff --git a/gdb/gdbtypes.h b/gdb/gdbtypes.h
index 75c77b3a90c..6faffb4217d 100644
--- a/gdb/gdbtypes.h
+++ b/gdb/gdbtypes.h
@@ -2209,6 +2209,8 @@ extern const struct floatformat *floatformats_ieee_double_littlebyte_bigword[BFD
 extern const struct floatformat *floatformats_i387_ext[BFD_ENDIAN_UNKNOWN];
 extern const struct floatformat *floatformats_m68881_ext[BFD_ENDIAN_UNKNOWN];
 extern const struct floatformat *floatformats_arm_ext[BFD_ENDIAN_UNKNOWN];
+extern const struct floatformat *floatformats_fp8_e5m2[BFD_ENDIAN_UNKNOWN];
+extern const struct floatformat *floatformats_fp8_e4m3[BFD_ENDIAN_UNKNOWN];
 extern const struct floatformat *floatformats_ia64_spill[BFD_ENDIAN_UNKNOWN];
 extern const struct floatformat *floatformats_vax_f[BFD_ENDIAN_UNKNOWN];
 extern const struct floatformat *floatformats_vax_d[BFD_ENDIAN_UNKNOWN];
diff --git a/gdb/target-descriptions.c b/gdb/target-descriptions.c
index dc5fbacfebd..9f9cc9ad95f 100644
--- a/gdb/target-descriptions.c
+++ b/gdb/target-descriptions.c
@@ -147,6 +147,16 @@ make_gdb_type (struct gdbarch *gdbarch, struct tdesc_type *ttype)
 	  m_type = init_float_type (alloc, -1, "builtin_type_bfloat16",
 				    floatformats_bfloat16);
 	  return;
+
+	case TDESC_TYPE_FP8_E5M2:
+	  m_type = init_float_type (alloc, -1, "builtin_type_fp8_e5m2",
+				    floatformats_fp8_e5m2);
+	  return;
+
+	case TDESC_TYPE_FP8_E4M3:
+	  m_type = init_float_type (alloc, -1, "builtin_type_fp8_e4m3",
+				    floatformats_fp8_e4m3);
+	  return;
 	}
 
       internal_error ("Type \"%s\" has an unknown kind %d",
diff --git a/gdb/target-float.c b/gdb/target-float.c
index 9b0376f2909..d0068e9f453 100644
--- a/gdb/target-float.c
+++ b/gdb/target-float.c
@@ -396,6 +396,12 @@ floatformat_classify (const struct floatformat *fmt,
 
   if (exponent == fmt->exp_nan)
     {
+      if (fmt->totalsize == 8 && fmt->exp_len == 4)
+	{
+	  if ((mant & 0b111) == 0b111)
+	    return float_nan;
+	  return float_normal;
+	}
       if (mant_zero)
 	return float_infinite;
       else
@@ -1287,7 +1293,11 @@ mpfr_float_ops::from_target (const struct floatformat *fmt,
   mant_off = fmt->man_start;
   mpfr_set_zero (to.val, 0);
 
-  special_exponent = exponent == 0 || exponent == fmt->exp_nan;
+  /* If format is FP8 E4M3, then we handle the case of NaN above, and there is
+     no inf.  */
+  special_exponent = (exponent == 0)
+		     || (!(fmt->totalsize == 8 && fmt->exp_len == 4)
+			 && (exponent == fmt->exp_nan));
 
   /* Don't bias NaNs.  Use minimum exponent for denorms.  For
      simplicity, we don't check for zero as the exponent doesn't matter.
diff --git a/gdbsupport/tdesc.cc b/gdbsupport/tdesc.cc
index ed92c864732..2b029484e51 100644
--- a/gdbsupport/tdesc.cc
+++ b/gdbsupport/tdesc.cc
@@ -54,7 +54,9 @@ static tdesc_type_builtin tdesc_predefined_types[] =
   { "ieee_double", TDESC_TYPE_IEEE_DOUBLE },
   { "arm_fpa_ext", TDESC_TYPE_ARM_FPA_EXT },
   { "i387_ext", TDESC_TYPE_I387_EXT },
-  { "bfloat16", TDESC_TYPE_BFLOAT16 }
+  { "bfloat16", TDESC_TYPE_BFLOAT16 },
+  { "fp8_e5m2", TDESC_TYPE_FP8_E5M2 },
+  { "fp8_e4m3", TDESC_TYPE_FP8_E4M3 }
 };
 
 void tdesc_feature::accept (tdesc_element_visitor &v) const
diff --git a/gdbsupport/tdesc.h b/gdbsupport/tdesc.h
index 6bf66ad3dcd..f898f49647e 100644
--- a/gdbsupport/tdesc.h
+++ b/gdbsupport/tdesc.h
@@ -176,6 +176,8 @@ enum tdesc_type_kind
   TDESC_TYPE_ARM_FPA_EXT,
   TDESC_TYPE_I387_EXT,
   TDESC_TYPE_BFLOAT16,
+  TDESC_TYPE_FP8_E5M2,
+  TDESC_TYPE_FP8_E4M3,
 
   /* Types defined by a target feature.  */
   TDESC_TYPE_VECTOR,
diff --git a/include/floatformat.h b/include/floatformat.h
index b65968a8755..dadb5a217aa 100644
--- a/include/floatformat.h
+++ b/include/floatformat.h
@@ -136,6 +136,11 @@ extern const struct floatformat floatformat_ibm_long_double_little;
 /* bfloat16.  */
 extern const struct floatformat floatformat_bfloat16_big;
 extern const struct floatformat floatformat_bfloat16_little;
+/* fp8.  */
+extern const struct floatformat floatformat_fp8_e5m2_big;
+extern const struct floatformat floatformat_fp8_e5m2_little;
+extern const struct floatformat floatformat_fp8_e4m3_big;
+extern const struct floatformat floatformat_fp8_e4m3_little;
 
 /* Convert from FMT to a double.
    FROM is the address of the extended float.
-- 
2.45.2