hardened conditionals

This patch introduces optional passes to harden conditionals used in
branches, and in computing boolean expressions, by adding redundant
tests of the reversed conditions, and trapping in case of unexpected
results.  Though in abstract machines the redundant tests should never
fail, CPUs may be led to misbehave under certain kinds of attacks,
such as of power deprivation, and these tests reduce the likelihood of
going too far down an unexpected execution path.


for  gcc/ChangeLog

	* common.opt (fharden-compares): New.
	(fharden-conditional-branches): New.
	* doc/invoke.texi: Document new options.
	* gimple-harden-conditionals.cc: New.
	* Makefile.in (OBJS): Build it.
	* passes.def: Add new passes.
	* tree-pass.h (make_pass_harden_compares): Declare.
	(make_pass_harden_conditional_branches): Declare.

for  gcc/ada/ChangeLog

	* doc/gnat_rm/security_hardening_features.rst
	(Hardened Conditionals): New.

for  gcc/testsuite/ChangeLog

	* c-c++-common/torture/harden-comp.c: New.
	* c-c++-common/torture/harden-cond.c: New.

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