The macro is documented like this in the internal manual:
-- Macro: WIDEST_HARDWARE_FP_SIZE
A C expression for the size in bits of the widest floating-point
format supported by the hardware. If you define this macro, you
must specify a value less than or equal to mode precision of the
mode used for C type 'long double' (from hook
'targetm.c.mode_for_floating_type' with argument
'TI_LONG_DOUBLE_TYPE'). If you do not define this macro, mode
precision of the mode used for C type 'long double' is the default.
AArch64 uses 128-bit TFmode for long double but, as far as I know, no FPU
implemented in hardware supports it.
gcc/
* config/aarch64/aarch64.h (WIDEST_HARDWARE_FP_SIZE): Define to 64.
gcc/testsuite/
* gnat.dg/specs/size_clause6.ads: New test.
#define LONG_LONG_TYPE_SIZE 64
+#define WIDEST_HARDWARE_FP_SIZE 64
+
/* This value is the amount of bytes a caller is allowed to drop the stack
before probing has to be done for stack clash protection. */
#define STACK_CLASH_CALLER_GUARD 1024
--- /dev/null
+-- { dg-do compile }
+
+with Interfaces;
+
+package Size_Clause6 is
+
+ type Long_Double is new Interfaces.IEEE_Extended_Float;
+ for Long_Double'Size use 128; -- { dg-warning "unused" "" { target { ! { { i?86-*-* x86_64-*-* } && lp64 } } } }
+
+ function Int (X : in Long_Double) return Integer is (0);
+
+end Size_Clause6;