Normally Boolean options/flags in GCC take the values zero or one.
This patch tweaks flag_trapping_math to take the values 0 or 65535.
More accurately it introduces a new trapping_math_model enumeration in
flag-types.h, and uses this to allow front-ends to (potentially) control
which expressions may be constant folded at compile-time by the middle-end.
Floating point/language experts may recognize these flags (bits) as being
modelled upon (extended) FENV_ACCESS.

This instalment simply introduces the necessary infrastructure without
yet changing any functionality.  The test "if (flag_trapping_math)" will
remain perfectly valid (but pessimistic).  The goal is to allow time
for out-of-tree front-ends (modula-2, rust, etc.) to update themselves,
if required, and to confirm that this change doesn't introduce problems
for LTO, or elsewhere.

This patch has been tested on x86_64-pc-linux-gnu with "make bootstrap"
and "make -k check", all languages including Ada, with no new failures.
Ok for mainline?


2021-09-25  Roger Sayle  <roger@nextmovesoftware.com>

gcc/ChangeLog
	* flag-types.h (trapping_math_model): New enumeration (of bits)
	specifying possible floating-point (and integer) exceptions/traps.
	* common.opt (ftrapping-math): Specify UInteger and initialize to
	flag_trapping_math to TRAPPING_MATH_DEFAULT.
	* toplev.c (process_options): The option -fsignaling-nans should
	set flag_trapping_math to TRAPPING_MATH_DEFAULT.

gcc/ada/ChangeLog
	* gcc-interface/misc.c (gnat_init_gcc_fp): Set flag_trapping_math
	to TRAPPING_MATH_DEFAULT (instead of 1) if S'Machine_Overflow.

Roger
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