uiltins: Add support for clang compatible __builtin_{add,sub}c{,l,ll} [PR79173]

While the design of these builtins in clang is questionable,
rather than being say
unsigned __builtin_addc (unsigned, unsigned, bool, bool *)
so that it is clear they add two [0, 0xffffffff] range numbers
plus one [0, 1] range carry in and give [0, 0xffffffff] range
return plus [0, 1] range carry out, they actually instead
add 3 [0, 0xffffffff] values together but the carry out
isn't then the expected [0, 2] value because
0xffffffffULL + 0xffffffff + 0xffffffff is 0x2fffffffd,
but just [0, 1] whether there was any overflow at all.

It is something used in the wild and shorter to write than the
corresponding
 #define __builtin_addc(a,b,carry_in,carry_out) \
  ({ unsigned _s; \
     unsigned _c1 = __builtin_uadd_overflow (a, b, &_s); \
     unsigned _c2 = __builtin_uadd_overflow (_s, carry_in, &_s); \
     *(carry_out) = (_c1 | _c2); \
     _s; })
and so a canned builtin for something people could often use.
It isn't that hard to maintain on the GCC side, as we just lower
it to two .ADD_OVERFLOW calls early, and the already committed
pottern recognization code can then make .UADDC/.USUBC calls out of
that if the carry in is in [0, 1] range and the corresponding
optab is supported by the target.

2023-06-16  Jakub Jelinek  <jakub@redhat.com>

	PR middle-end/79173
	* builtin-types.def (BT_FN_UINT_UINT_UINT_UINT_UINTPTR,
	BT_FN_ULONG_ULONG_ULONG_ULONG_ULONGPTR,
	BT_FN_ULONGLONG_ULONGLONG_ULONGLONG_ULONGLONG_ULONGLONGPTR): New
	types.
	* builtins.def (BUILT_IN_ADDC, BUILT_IN_ADDCL, BUILT_IN_ADDCLL,
	BUILT_IN_SUBC, BUILT_IN_SUBCL, BUILT_IN_SUBCLL): New builtins.
	* builtins.cc (fold_builtin_addc_subc): New function.
	(fold_builtin_varargs): Handle BUILT_IN_{ADD,SUB}C{,L,LL}.
	* doc/extend.texi (__builtin_addc, __builtin_subc): Document.

	* gcc.target/i386/pr79173-11.c: New test.
	* gcc.dg/builtin-addc-1.c: New test.

1 files changed, 44 insertions, 0 deletions

diff --git a/gcc/doc/extend.texi b/gcc/doc/extend.texi
index e01e244..c01cd3f 100644
--- a/gcc/doc/extend.texi
+++ b/gcc/doc/extend.texi
@@ -12839,6 +12839,50 @@ after addition, conditional jump on carry etc.
  
 @enddefbuiltin
 
+@defbuiltin{unsigned int __builtin_addc (unsigned int a, unsigned int b, unsigned int carry_in, unsigned int *carry_out)}
+@defbuiltinx{unsigned long int __builtin_addcl (unsigned long int a, unsigned long int b, unsigned int carry_in, unsigned long int *carry_out)}
+@defbuiltinx{unsigned long long int __builtin_addcll (unsigned long long int a, unsigned long long int b, unsigned long long int carry_in, unsigned long long int *carry_out)}
+
+These built-in functions are equivalent to:
+@smallexample
+  (@{ __typeof__ (@var{a}) s; \
+      __typeof__ (@var{a}) c1 = __builtin_add_overflow (@var{a}, @var{b}, &s); \
+      __typeof__ (@var{a}) c2 = __builtin_add_overflow (s, @var{carry_in}, &s); \
+      *(@var{carry_out}) = c1 | c2; \
+      s; @})
+@end smallexample
+
+i.e.@: they add 3 unsigned values, set what the last argument
+points to to 1 if any of the two additions overflowed (otherwise 0)
+and return the sum of those 3 unsigned values.  Note, while all
+the first 3 arguments can have arbitrary values, better code will be
+emitted if one of them (preferrably the third one) has only values
+0 or 1 (i.e.@: carry-in).
+
+@enddefbuiltin
+
+@defbuiltin{unsigned int __builtin_subc (unsigned int a, unsigned int b, unsigned int carry_in, unsigned int *carry_out)}
+@defbuiltinx{unsigned long int __builtin_subcl (unsigned long int a, unsigned long int b, unsigned int carry_in, unsigned long int *carry_out)}
+@defbuiltinx{unsigned long long int __builtin_subcll (unsigned long long int a, unsigned long long int b, unsigned long long int carry_in, unsigned long long int *carry_out)}
+
+These built-in functions are equivalent to:
+@smallexample
+  (@{ __typeof__ (@var{a}) s; \
+      __typeof__ (@var{a}) c1 = __builtin_sub_overflow (@var{a}, @var{b}, &s); \
+      __typeof__ (@var{a}) c2 = __builtin_sub_overflow (s, @var{carry_in}, &s); \
+      *(@var{carry_out}) = c1 | c2; \
+      s; @})
+@end smallexample
+
+i.e.@: they subtract 2 unsigned values from the first unsigned value,
+set what the last argument points to to 1 if any of the two subtractions
+overflowed (otherwise 0) and return the result of the subtractions.
+Note, while all the first 3 arguments can have arbitrary values, better code
+will be emitted if one of them (preferrably the third one) has only values
+0 or 1 (i.e.@: carry-in).
+
+@enddefbuiltin
+
 @node x86 specific memory model extensions for transactional memory
 @section x86-Specific Memory Model Extensions for Transactional Memory