testsuite: Add new target check for no_alignment_constraints

A few testcases were marked for avr target, which has no alignment
requirements.  But those tests in fact should filter for any
target having __BIGGEST_ALIGNMENT__=1.

A new effective target check is introduced: no_alignment_constraints.
It checks whether __BIGGEST_ALIGNMENT__ is declared as 1.

This change fixes the testsuite cases for PRU target.

It was regression-tested on x86_64-pc-linux-gnu.

The following two existing macros were considered, but they check for
subtly different target behaviour:
 1. non_strict_align
    If true, non-aligned access is permitted. But it also allows
    variables to be naturally aligned, which is not true for
    no_alignment_constraints.

 2. default_packed
    Whether structures are packed by default is not necessarily
    the same as lacking constraints for non-aggregate types.
    For example, BIGGEST_FIELD_ALIGNMENT or ADJUST_FIELD_ALIGN
    could be defined for a target as something other than
    BIGGEST_ALIGNMENT.

gcc/ChangeLog:

	* doc/sourcebuild.texi: Document new no_alignment_constraints
	effective target check.

gcc/testsuite/ChangeLog:

	* c-c++-common/Wcast-align.c: Silence warnings for targets with
	no_alignment_constraints.
	* gcc.dg/c11-align-4.c: Skip for no_alignment_constraints.
	* gcc.dg/strlenopt-10.c: Replace checks for avr with checks for
	any target with no_alignment_constraints.
	* gcc.dg/strlenopt-11.c: Ditto.
	* gcc.dg/strlenopt-13.c: Ditto.
	* lib/target-supports.exp
	(check_effective_target_no_alignment_constraints): New.

Signed-off-by: Dimitar Dimitrov <dimitar@dinux.eu>

1 files changed, 9 insertions, 0 deletions

diff --git a/gcc/doc/sourcebuild.texi b/gcc/doc/sourcebuild.texi
index 3696a58..760ff95 100644
--- a/gcc/doc/sourcebuild.texi
+++ b/gcc/doc/sourcebuild.texi
@@ -2716,6 +2716,7 @@ Target supports @option{-branch-cost=N}.
 Target uses @code{__cxa_atexit}.
 
 @item default_packed
+@anchor{default_packed}
 Target has packed layout of structure members by default.
 
 @item exceptions
@@ -2786,6 +2787,14 @@ Target uses natural alignment (aligned to type size) for types of
 Target uses natural alignment (aligned to type size) for types of
 64 bits or less.
 
+@item no_alignment_constraints
+Target defines __BIGGEST_ALIGNMENT__=1.  Hence target imposes
+no alignment constraints.  This is similar, but not necessarily
+the same as @ref{default_packed}.  Although @code{BIGGEST_FIELD_ALIGNMENT}
+defaults to @code{BIGGEST_ALIGNMENT} for most targets, it is possible
+for a target to set those two with different values and have different
+alignment constraints for aggregate and non-aggregate types.
+
 @item noinit
 Target supports the @code{noinit} variable attribute.