Fix linemap corruption after very wide source lines (PR c++/72803)

PR c++/72803 describes an issue where a fix-it hint is to be emitted at
column 512 of a 511-column source line, leading to an ICE.

The root cause is a bug in linemap_line_start, when transitioning from
lines >= 512 in width to narrow lines.

The wide line in the reproducer has a line map with:
  m_column_and_range_bits = 15, m_range_bits = 5
giving 10 effective bits for representing columns, so that columns <= 1023
can be represented.

When parsing the following line,
  linemap_line_start (..., ..., max_column_hint=0);
is called.  This leads to the "add_map" logic, due to this condition:
      || (max_column_hint <= 80 && effective_column_bits >= 10)
i.e. the new line is sufficiently narrower than the old one to
potentially use a new linemap (so as to conserve values within the
location_t space).

It then attempts to avoid allocating a new line map.  Part of the logic
to determine if we really need a new line map is this condition:
   SOURCE_COLUMN (map, highest) >= (1U << column_bits)
The above condition is incorrect: we need to determine if the highest
column we've handed out will fit within the proposed *effective* column
bits, but "column_bits" here is the column plus the range bits, rather
than just the column bits.

Hence in this case linemap_line_start erroneously decides that we don't
need a new line map, and updates the column bits within the existing
line map, so any location_t values we've already handed out within it
that are offset from the start by
  >= (1<<new_column_and_range_bits)
effectively change meaning, leading to incorrect line&column information
when decoding them, and various "interesting" ways for the linemap
code to fail.

The fix is to use the effective column bits in the above conditional.

gcc/ChangeLog:
	PR c++/72803
	* input.c (selftest::test_accessing_ordinary_linemaps): Verify
	that the transition from a max line width >= 1<<10 to narrower
	lines works correctly.

gcc/testsuite/ChangeLog:
	PR c++/72803
	* g++.dg/diagnostic/pr72803.C: New test case.

libcpp/ChangeLog:
	PR c++/72803
	* line-map.c (linemap_line_start): When determining if the highest
	column given out so far will fit into a proposed change to the
	current map, use the effective number of column bits, rather than
	the total number of column + range bits.



git-svn-id: svn+ssh://gcc.gnu.org/svn/gcc/trunk@244199 138bc75d-0d04-0410-961f-82ee72b054a4

1 files changed, 8 insertions, 0 deletions

diff --git a/libcpp/ChangeLog b/libcpp/ChangeLog
index e9055bb92a4..082291d5648 100644
--- a/libcpp/ChangeLog
+++ b/libcpp/ChangeLog
@@ -1,3 +1,11 @@
+2017-01-07  David Malcolm  <dmalcolm@redhat.com>
+
+	PR c++/72803
+	* line-map.c (linemap_line_start): When determining if the highest
+	column given out so far will fit into a proposed change to the
+	current map, use the effective number of column bits, rather than
+	the total number of column + range bits.
+
 2017-01-01  Jakub Jelinek  <jakub@redhat.com>
 
 	Update copyright years.