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compat/obstack: fix -Wcast-function-type warnings

GCC 8 introduced the new -Wcast-function-type warning, which is
implied by -Wextra (which, in turn is enabled in our DEVELOPER flags).
When building Git with GCC 8 and this warning enabled on a non-glibc
platform [1], one is greeted with a screenful of compiler
warnings/errors:

compat/obstack.c: In function '_obstack_begin':
compat/obstack.c:162:17: error: cast between incompatible function types from 'void * (*)(long int)' to 'struct _obstack_chunk * (*)(void *, long int)' [-Werror=cast-function-type]
h->chunkfun = (struct _obstack_chunk * (*)(void *, long)) chunkfun;
^
compat/obstack.c:163:16: error: cast between incompatible function types from 'void (*)(void *)' to 'void (*)(void *, struct _obstack_chunk *)' [-Werror=cast-function-type]
h->freefun = (void (*) (void *, struct _obstack_chunk *)) freefun;
^
compat/obstack.c:116:8: error: cast between incompatible function types from 'struct _obstack_chunk * (*)(void *, long int)' to 'struct _obstack_chunk * (*)(long int)' [-Werror=cast-function-type]
: (*(struct _obstack_chunk *(*) (long)) (h)->chunkfun) ((size)))
^
compat/obstack.c:168:22: note: in expansion of macro 'CALL_CHUNKFUN'
chunk = h->chunk = CALL_CHUNKFUN (h, h -> chunk_size);
^~~~~~~~~~~~~
<snip>

'struct obstack' stores pointers to two functions to allocate and free
"chunks", and depending on how obstack is used, these functions take
either one parameter (like standard malloc() and free() do; this is
how we use it in 'kwset.c') or two parameters. Presumably to reduce
memory footprint, a single field is used to store the function pointer
for both signatures, and then it's casted to the appropriate signature
when the function pointer is accessed. These casts between function
pointers with different number of parameters are what trigger those
compiler errors.

Modify 'struct obstack' to use unions to store function pointers with
different signatures, and then use the union member with the
appropriate signature when accessing these function pointers. This
eliminates the need for those casts, and thus avoids this compiler
error.

[1] Compiling 'compat/obstack.c' on a platform with glibc is sort of
a noop, see the comment before '# define ELIDE_CODE', so this is
not an issue on common Linux distros.

Signed-off-by: SZEDER Gábor <szeder.dev@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

authored by

SZEDER Gábor and committed by
Junio C Hamano 764473d2 268fbcd1

+20 -15
unified split
+9 -8
compat/obstack.c
··· 112 112 113 113 # define CALL_CHUNKFUN(h, size) \ 114 114 (((h) -> use_extra_arg) \ 115 - ? (*(h)->chunkfun) ((h)->extra_arg, (size)) \ 116 - : (*(struct _obstack_chunk *(*) (long)) (h)->chunkfun) ((size))) 115 + ? (*(h)->chunkfun.extra) ((h)->extra_arg, (size)) \ 116 + : (*(h)->chunkfun.plain) ((size))) 117 117 118 118 # define CALL_FREEFUN(h, old_chunk) \ 119 119 do { \ 120 120 if ((h) -> use_extra_arg) \ 121 - (*(h)->freefun) ((h)->extra_arg, (old_chunk)); \ 121 + (*(h)->freefun.extra) ((h)->extra_arg, (old_chunk)); \ 122 122 else \ 123 - (*(void (*) (void *)) (h)->freefun) ((old_chunk)); \ 123 + (*(h)->freefun.plain) ((old_chunk)); \ 124 124 } while (0) 125 125 126 126 ··· 159 159 size = 4096 - extra; 160 160 } 161 161 162 - h->chunkfun = (struct _obstack_chunk * (*)(void *, long)) chunkfun; 163 - h->freefun = (void (*) (void *, struct _obstack_chunk *)) freefun; 162 + h->chunkfun.plain = chunkfun; 163 + h->freefun.plain = freefun; 164 164 h->chunk_size = size; 165 165 h->alignment_mask = alignment - 1; 166 166 h->use_extra_arg = 0; ··· 206 206 size = 4096 - extra; 207 207 } 208 208 209 - h->chunkfun = (struct _obstack_chunk * (*)(void *,long)) chunkfun; 210 - h->freefun = (void (*) (void *, struct _obstack_chunk *)) freefun; 209 + h->chunkfun.extra = (struct _obstack_chunk * (*)(void *,long)) chunkfun; 210 + h->freefun.extra = (void (*) (void *, struct _obstack_chunk *)) freefun; 211 + 211 212 h->chunk_size = size; 212 213 h->alignment_mask = alignment - 1; 213 214 h->extra_arg = arg;
+11 -7
compat/obstack.h
··· 160 160 void *tempptr; 161 161 } temp; /* Temporary for some macros. */ 162 162 int alignment_mask; /* Mask of alignment for each object. */ 163 - /* These prototypes vary based on `use_extra_arg', and we use 164 - casts to the prototypeless function type in all assignments, 165 - but having prototypes here quiets -Wstrict-prototypes. */ 166 - struct _obstack_chunk *(*chunkfun) (void *, long); 167 - void (*freefun) (void *, struct _obstack_chunk *); 163 + /* These prototypes vary based on `use_extra_arg'. */ 164 + union { 165 + void *(*plain) (long); 166 + struct _obstack_chunk *(*extra) (void *, long); 167 + } chunkfun; 168 + union { 169 + void (*plain) (void *); 170 + void (*extra) (void *, struct _obstack_chunk *); 171 + } freefun; 168 172 void *extra_arg; /* first arg for chunk alloc/dealloc funcs */ 169 173 unsigned use_extra_arg:1; /* chunk alloc/dealloc funcs take extra arg */ 170 174 unsigned maybe_empty_object:1;/* There is a possibility that the current ··· 235 239 (void (*) (void *, void *)) (freefun), (arg)) 236 240 237 241 #define obstack_chunkfun(h, newchunkfun) \ 238 - ((h) -> chunkfun = (struct _obstack_chunk *(*)(void *, long)) (newchunkfun)) 242 + ((h)->chunkfun.extra = (struct _obstack_chunk *(*)(void *, long)) (newchunkfun)) 239 243 240 244 #define obstack_freefun(h, newfreefun) \ 241 - ((h) -> freefun = (void (*)(void *, struct _obstack_chunk *)) (newfreefun)) 245 + ((h)->freefun.extra = (void (*)(void *, struct _obstack_chunk *)) (newfreefun)) 242 246 243 247 #define obstack_1grow_fast(h,achar) (*((h)->next_free)++ = (achar)) 244 248