Clarify C type sizes and alignments

I have found the specicifaction confusing, because C type sizes and
alignments have so far been defined only for the *architectures* RV32G
and RV64G. This is confusing, as there are 32- and 64-bit architectures
without all parts of the G extension which also share these type sizes
and alignments.

This commit separates the definitions of C type sizes and alignments
into new section, and defines them for particular ABIs, not
architectures. The definitions themselves remain largely unchanged,
though wording has been clarified and extra subsection headers have been
introduced.

For the avoidance of doubt around C struct alignment rules, I have
defined the RISC-V psABI rules around size and alignment of structures
and unions. This is based on both the x86-64 and Intel386 System V psABI

This now explicitly enumerates the sizes and alignments of C11's complex
types into the tables of sizes and alignments, based on

Author: lenary

riscv-elf.md

@@ -241,72 +241,93 @@ s0-s1, and only three temporaries, t0-t2.
 If used with an ISA that has any of the registers x16-x31 and f0-f31, then
 these registers are considered temporaries.
 
-## <a name=default-abis-and-c-type-sizes></a> Default ABIs and C type sizes
+## <a name=default-abis></a> Default ABIs
 
 While various different ABIs are technically possible, for software
 compatibility reasons it is strongly recommended to use the following
-default ABIs:
-
-  * **on RV64G**: LP64 with floats and doubles passed in floating point
-    registers, i.e. ELFCLASS64 and EF_RISCV_FLOAT_ABI_DOUBLE, using the
-    following C type sizes:
-
-    Type        | Size (Bytes)  | Alignment (Bytes)
-    ------------|---------------|------------------
-    bool/_Bool  |  1            |  1
-    char        |  1            |  1
-    short       |  2            |  2
-    int         |  4            |  4
-    wchar_t     |  4            |  4
-    wint_t      |  4            |  4
-    long        |  8            |  8
-    long long   |  8            |  8
-    __int128    | 16            | 16
-    void *      |  8            |  8
-    float       |  4            |  4
-    double      |  8            |  8
-    long double | 16            | 16
+default ABIs for specific architectures:
+
+  * **on RV64G**: LP64D for XLEN=64, with floats and doubles passed in floating
+    point registers (i.e. ELFCLASS64 and EF_RISCV_FLOAT_ABI_DOUBLE).
 
     Although RV64GQ systems can technically use EF_RISCV_FLOAT_ABI_QUAD,
     it is strongly recommended to use EF_RISCV_FLOAT_ABI_DOUBLE on
     general-purpose RV64GQ systems for compatibility with standard RV64G
     software.
 
-  * **on RV32G**: ILP32 with floats and doubles passed in floating point
-    registers, i.e. ELFCLASS32 and EF_RISCV_FLOAT_ABI_DOUBLE, using the
-    following C type sizes:
-
-    Type        | Size (Bytes)  | Alignment (Bytes)
-    ------------|---------------|------------------
-    bool/_Bool  |  1            |  1
-    char        |  1            |  1
-    short       |  2            |  2
-    int         |  4            |  4
-    wchar_t     |  4            |  4
-    wint_t      |  4            |  4
-    long        |  4            |  4
-    long long   |  8            |  8
-    void *      |  4            |  4
-    float       |  4            |  4
-    double      |  8            |  8
-    long double | 16            | 16
-
-`char` is unsigned.  `wchar_t` is signed.  `wint_t` is unsigned.
-
-`_Complex` types have the alignment and layout of a struct containing two
-fields of the corresponding real type (`float`, `double`, or `long double`),
-with the first field holding the real part and the second field holding
-the imaginary part.
+  * **on RV32G**: ILP32D for XLEN=32, with floats and doubles passed in floating
+    point registers (i.e. ELFCLASS32 and EF_RISCV_FLOAT_ABI_DOUBLE).
 
 A future version of this specification may define an ILP32 ABI for
 RV64G, but currently this is not a supported operating mode.
 
+# <a name=c-types></a> C type details
+## <a name=c-type-sizes></a> C type sizes and alignments
+
+There are two conventions for C type sizes and alignments.
+
+  * **LP64, LP64F, and LP64D**: use the following type sizes and alignments
+    (based on the LP64 convention):
+
+    Type                 | Size (Bytes)  | Alignment (Bytes)
+    ---------------------|---------------|------------------
+    bool/_Bool           |  1            |  1
+    char                 |  1            |  1
+    short                |  2            |  2
+    int                  |  4            |  4
+    wchar_t              |  4            |  4
+    wint_t               |  4            |  4
+    long                 |  8            |  8
+    long long            |  8            |  8
+    __int128             | 16            | 16
+    void *               |  8            |  8
+    float                |  4            |  4
+    double               |  8            |  8
+    long double          | 16            | 16
+    float _Complex       |  8            |  4
+    double _Complex      | 16            |  8
+    long double _Complex | 32            | 16
+
+  * **ILP32, ILP32F, and ILP32D**: use the following type sizes and alignments
+    (based on the ILP32 convention):
+
+    Type                 | Size (Bytes)  | Alignment (Bytes)
+    ---------------------|---------------|------------------
+    bool/_Bool           |  1            |  1
+    char                 |  1            |  1
+    short                |  2            |  2
+    int                  |  4            |  4
+    wchar_t              |  4            |  4
+    wint_t               |  4            |  4
+    long                 |  4            |  4
+    long long            |  8            |  8
+    void *               |  4            |  4
+    float                |  4            |  4
+    double               |  8            |  8
+    long double          | 16            | 16
+    float _Complex       |  8            |  4
+    double _Complex      | 16            |  8
+    long double _Complex | 32            | 16
+
+The alignment of `max_align_t` is 16.
+
+Structs and unions are aligned to the alignment of their most strictly aligned
+member. The size of any object is a multiple of its alignment.
+
+## <a name=c-type-signedness></a> C type signedness
+
+`char` is unsigned.  `wchar_t` is signed.  `wint_t` is unsigned.
+
+## <a name=c-type-representation></a> C type values
+
 Booleans (`bool`/`_Bool`) stored in memory or when being passed as scalar
 arguments are either `0` (`false`) or `1` (`true`).
 
 A null pointer (for all types) has the value zero.
 
-The value of `_Alignof(max_align_t)` is 16.
+`_Complex` types have the layout of a struct containing two fields of the
+corresponding real type (`float`, `double`, or `long double`), with the first
+field holding the real part and the second field holding the imaginary part.
 
 ## <a name=va-list-va-start-and-va-arg></a> va_list, va_start, and va_arg