committed: improved unreachable code warning

[Arnaud Charlet <charlet at aix dot act-europe dot fr>]

Tested on i686-linux.

The compiler already had circuitry to detect whether every exit
branch of an IF or CASE was terminated by an unconditional
transfer of control. If so, the statement after the IF or CASE
is dead code, and is diagnosed as unreachable. However, in the
case of a block, no such analysis was done. This patch implements
exactly parallel handling for blocks. Now, all exit paths from a
block are checked (there can be more than one because of exception
handlers), and if all are terminated by an unconditional transfer
of control, then the statement after the block is unreachable.
The treatment of blocks is exactly parallel to the treatment of
IF and CASE, so no new mechanism was required, it was just a
matter of using this existing mechanism for blocks.
branch of an IF or CASE was terminated by an unconditional
transfer of control. If so, the statement after the IF or CASE
is dead code, and is diagnosed as unreachable. However, in the
case of a block, no such analysis was done. This patch implements
exactly parallel handling for blocks. Now, all exit paths from a
block are checked (there can be more than one because of exception
handlers), and if all are terminated by an unconditional transfer
of control, then the statement after the block is unreachable.
The treatment of blocks is exactly parallel to the treatment of
IF and CASE, so no new mechanism was required, it was just a
matter of using this existing mechanism for blocks.

2004-10-04  Robert Dewar  <dewar@gnat.com>

	* sem_ch5.adb (Unblocked_Exit_Count): Now used for blocks as well as
	IF and CASE.
	(Analyze_Block_Statement): Add circuitry to detect following dead code
	(Check_Unreachable_Code): Handle case of block exit

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