gdb/riscv: Split ISA and ABI features

The goal of this commit is to allow RV64 binaries compiled for the 'F'
extension to run on a target that supports both the 'F' and 'D'
extensions.

The 'D' extension depends on the 'F' extension and chapter 9 of the
RISC-V ISA manual implies that running a program compiled for 'F' on
a 'D' target should be fine.

To support this the gdbarch now holds two feature sets, one represents
the features that are present on the target, and one represents the
features requested in the ELF flags.

The existing error checks are relaxed slightly to allow binaries
compiled for 32-bit 'F' extension to run on targets with the 64-bit
'D' extension.

A new set of functions called riscv_abi_{xlen,flen} are added to
compliment the existing riscv_isa_{xlen,flen}, and some callers to the
isa functions now call the abi functions when that is appropriate.

In riscv_call_arg_struct two asserts are removed, these asserts no
longer make sense.  The asserts were both like this:

    gdb_assert (TYPE_LENGTH (ainfo->type)
                <= (cinfo->flen + cinfo->xlen));

And were made in two cases, when passing structures like these:

   struct {
     integer field1;
     float   field2;
   };

or,

   struct {
     float   field1;
     integer field2;
   };

When running on an RV64 target which only has 32-bit float then the
integer field could be 64-bits, while if the float field is 32-bits
the overall size of the structure can be 128-bits (with 32-bits of
padding).  In this case the assertion would fail, however, the code
isn't incorrect, so its safe to just remove the assertion.

This was tested by running on an RV64IMFDC target using a compiler
configured for RV64IMFC, and comparing the results with those obtained
when using a compiler configured for RV64IMFDC.  The only regressions
I see (now) are in gdb.base/store.exp and are related too different
code generation choices GCC makes between the two targets.

Finally, this commit does not make any attempt to support running
binaries compiled for RV32 on an RV64 target, though nothing in here
should prevent that being supported in the future.

gdb/ChangeLog:

	* arch/riscv.h (struct riscv_gdbarch_features) <hw_float_abi>:
	Delete.
	<operator==>: Update with for removed field.
	<hash>: Likewise.
	* riscv-tdep.h (struct gdbarch_tdep) <features>: Renamed to...
	<isa_features>: ...this.
	<abi_features>: New field.
	(riscv_isa_flen): Update comment.
	(riscv_abi_xlen): New declaration.
	(riscv_abi_flen): New declaration.
	* riscv-tdep.c (riscv_isa_xlen): Update to get answer from
	isa_features.
	(riscv_abi_xlen): New function.
	(riscv_isa_flen): Update to get answer from isa_features.
	(riscv_abi_flen): New function.
	(riscv_has_fp_abi): Update to get answer from abi_features.
	(riscv_call_info::riscv_call_info): Use abi xlen and flen, not isa
	xlen and flen.
	(riscv_call_info) <xlen, flen>: Update comment.
	(riscv_call_arg_struct): Remove invalid assertions
	(riscv_features_from_gdbarch_info): Update now hw_float_abi field
	is removed.
	(riscv_gdbarch_init): Gather isa features and abi features
	separately, ensure both match on the gdbarch when reusing an old
	gdbarch.  Relax an error check to allow 32-bit abi float to run on
	a target with 64-bit float hardware.

1 files changed, 44 insertions, 37 deletions

diff --git a/gdb/riscv-tdep.c b/gdb/riscv-tdep.c
index ac3dd38..33c1c44 100644
--- a/gdb/riscv-tdep.c
+++ b/gdb/riscv-tdep.c
@@ -361,7 +361,15 @@ static unsigned int riscv_debug_gdbarch = 0;
 int
 riscv_isa_xlen (struct gdbarch *gdbarch)
 {
-  return gdbarch_tdep (gdbarch)->features.xlen;
+  return gdbarch_tdep (gdbarch)->isa_features.xlen;
+}
+
+/* See riscv-tdep.h.  */
+
+int
+riscv_abi_xlen (struct gdbarch *gdbarch)
+{
+  return gdbarch_tdep (gdbarch)->abi_features.xlen;
 }
 
 /* See riscv-tdep.h.  */
@@ -369,7 +377,15 @@ riscv_isa_xlen (struct gdbarch *gdbarch)
 int
 riscv_isa_flen (struct gdbarch *gdbarch)
 {
-  return gdbarch_tdep (gdbarch)->features.flen;
+  return gdbarch_tdep (gdbarch)->isa_features.flen;
+}
+
+/* See riscv-tdep.h.  */
+
+int
+riscv_abi_flen (struct gdbarch *gdbarch)
+{
+  return gdbarch_tdep (gdbarch)->abi_features.flen;
 }
 
 /* Return true if the target for GDBARCH has floating point hardware.  */
@@ -385,7 +401,7 @@ riscv_has_fp_regs (struct gdbarch *gdbarch)
 static bool
 riscv_has_fp_abi (struct gdbarch *gdbarch)
 {
-  return gdbarch_tdep (gdbarch)->features.hw_float_abi;
+  return gdbarch_tdep (gdbarch)->abi_features.flen > 0;
 }
 
 /* Return true if REGNO is a floating pointer register.  */
@@ -1786,8 +1802,8 @@ struct riscv_call_info
     : int_regs (RISCV_A0_REGNUM, RISCV_A0_REGNUM + 7),
       float_regs (RISCV_FA0_REGNUM, RISCV_FA0_REGNUM + 7)
   {
-    xlen = riscv_isa_xlen (gdbarch);
-    flen = riscv_isa_flen (gdbarch);
+    xlen = riscv_abi_xlen (gdbarch);
+    flen = riscv_abi_flen (gdbarch);
 
     /* Disable use of floating point registers if needed.  */
     if (!riscv_has_fp_abi (gdbarch))
@@ -1807,7 +1823,7 @@ struct riscv_call_info
   struct riscv_arg_reg float_regs;
 
   /* The XLEN and FLEN are copied in to this structure for convenience, and
-     are just the results of calling RISCV_ISA_XLEN and RISCV_ISA_FLEN.  */
+     are just the results of calling RISCV_ABI_XLEN and RISCV_ABI_FLEN.  */
   int xlen;
   int flen;
 };
@@ -2148,9 +2164,6 @@ riscv_call_arg_struct (struct riscv_arg_info *ainfo,
 	{
 	  int len0, len1, offset;
 
-	  gdb_assert (TYPE_LENGTH (ainfo->type)
-		      <= (cinfo->flen + cinfo->xlen));
-
 	  len0 = TYPE_LENGTH (sinfo.field_type (0));
 	  if (!riscv_assign_reg_location (&ainfo->argloc[0],
 					  &cinfo->float_regs, len0, 0))
@@ -2174,9 +2187,6 @@ riscv_call_arg_struct (struct riscv_arg_info *ainfo,
 	{
 	  int len0, len1, offset;
 
-	  gdb_assert (TYPE_LENGTH (ainfo->type)
-		      <= (cinfo->flen + cinfo->xlen));
-
 	  len0 = TYPE_LENGTH (sinfo.field_type (0));
 	  len1 = TYPE_LENGTH (sinfo.field_type (1));
 	  offset = align_up (len0, riscv_type_alignment (sinfo.field_type (1)));
@@ -2889,15 +2899,9 @@ riscv_features_from_gdbarch_info (const struct gdbarch_info info)
 			_("unknown ELF header class %d"), eclass);
 
       if (e_flags & EF_RISCV_FLOAT_ABI_DOUBLE)
-	{
-	  features.flen = 8;
-	  features.hw_float_abi = true;
-	}
+	features.flen = 8;
       else if (e_flags & EF_RISCV_FLOAT_ABI_SINGLE)
-	{
-	  features.flen = 4;
-	  features.hw_float_abi = true;
-	}
+	features.flen = 4;
     }
   else
     {
@@ -3115,25 +3119,26 @@ riscv_gdbarch_init (struct gdbarch_info info,
   /* Have a look at what the supplied (if any) bfd object requires of the
      target, then check that this matches with what the target is
      providing.  */
-  struct riscv_gdbarch_features info_features
+  struct riscv_gdbarch_features abi_features
     = riscv_features_from_gdbarch_info (info);
-  if (info_features.xlen != 0 && info_features.xlen != features.xlen)
+  /* In theory a binary compiled for RV32 could run on an RV64 target,
+     however, this has not been tested in GDB yet, so for now we require
+     that the requested xlen match the targets xlen.  */
+  if (abi_features.xlen != 0 && abi_features.xlen != features.xlen)
     error (_("bfd requires xlen %d, but target has xlen %d"),
-           info_features.xlen, features.xlen);
-  if (info_features.flen != 0 && info_features.flen != features.flen)
+            abi_features.xlen, features.xlen);
+  /* We do support running binaries compiled for 32-bit float on targets
+     with 64-bit float, so we only complain if the binary requires more
+     than the target has available.  */
+  if (abi_features.flen > features.flen)
     error (_("bfd requires flen %d, but target has flen %d"),
-           info_features.flen, features.flen);
-
-  /* If the xlen from INFO_FEATURES is 0 then this indicates either there
-     is no bfd object, or nothing useful could be extracted from it, in
-     this case we enable hardware float abi if the target has floating
-     point registers.
+            abi_features.flen, features.flen);
 
-     If the xlen from INFO_FEATURES is not 0, and the flen in
-     INFO_FEATURES is also not 0, then this indicates that the supplied
-     bfd does require hardware floating point abi.  */
-  if (info_features.xlen == 0 || info_features.flen != 0)
-    features.hw_float_abi = (features.flen > 0);
+  /* If the ABI_FEATURES xlen is 0 then this indicates we got no useful abi
+     features from the INFO object.  In this case we assume that the xlen
+     abi matches the hardware.  */
+  if (abi_features.xlen == 0)
+    abi_features.xlen = features.xlen;
 
   /* Find a candidate among the list of pre-declared architectures.  */
   for (arches = gdbarch_list_lookup_by_info (arches, &info);
@@ -3145,7 +3150,8 @@ riscv_gdbarch_init (struct gdbarch_info info,
          gdbarch.  */
       struct gdbarch_tdep *other_tdep = gdbarch_tdep (arches->gdbarch);
 
-      if (other_tdep->features != features)
+      if (other_tdep->isa_features != features
+	  || other_tdep->abi_features != abi_features)
         continue;
 
       break;
@@ -3160,7 +3166,8 @@ riscv_gdbarch_init (struct gdbarch_info info,
   /* None found, so create a new architecture from the information provided.  */
   tdep = new (struct gdbarch_tdep);
   gdbarch = gdbarch_alloc (&info, tdep);
-  tdep->features = features;
+  tdep->isa_features = features;
+  tdep->abi_features = abi_features;
 
   /* Target data types.  */
   set_gdbarch_short_bit (gdbarch, 16);