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.
From-SVN: r244199
5ccf1d8d10