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sizeof(Tile) == 10 rather than the expected 8 bytes. #6218

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DorpsGek opened this issue Jan 24, 2015 · 3 comments
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sizeof(Tile) == 10 rather than the expected 8 bytes. #6218

DorpsGek opened this issue Jan 24, 2015 · 3 comments
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flyspray This issue is imported from FlySpray (https://bugs.openttd.org/)

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@DorpsGek
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bcmpinc opened the ticket and wrote:

The Tile data type has a size of 10 bytes, even though the commit log suggests that it should be 8 bytes. This is caused by the 2-byte member 'm2', which is on an alignement boundary, as is shown below. As a result, the map wastes 2 bytes per tile. This was introduced by commit 26873 and #6510 tried to reduce the size of Tile back to 8 bytes, which failed due to the position of m2 in the struct.

The current (r27120) definition of Tile in map_type.h:

struct Tile {
byte type; ///< The type (bits 4..7), bridges (2..3), rainforest/desert (0..1)
byte height; ///< The height of the northern corner.
byte m1; ///< Primarily used for ownership information
uint16 m2; ///< Primarily used for indices to towns, industries and stations
byte m3; ///< General purpose
byte m4; ///< General purpose
byte m5; ///< General purpose
};

Reported version: trunk
Operating system: All

This issue was imported from FlySpray: https://bugs.openttd.org/task/6218
@DorpsGek
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planetmaker wrote:

I count 6 bytes and one uint16 which is two bytes, thus the total is eight bytes. Where is now the issue?

This comment was imported from FlySpray: https://bugs.openttd.org/task/6218#comment13744

@DorpsGek
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bcmpinc wrote:

After m1 the compiler will insert a dummy-byte, to ensure that m2 is properly aligned. After m5 another dummy byte is added also for alignment purposes. It is probably best demonstrated with a test program. So I've attached a small program that you can compile and run to see that the current definition of Tile takes up 10 bytes (and thus is wasting 2 bytes). The problem can be fixed by moving m2 above m1.

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This comment was imported from FlySpray: https://bugs.openttd.org/task/6218#comment13745

@DorpsGek
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planetmaker closed the ticket.

Reason for closing: Fixed

In r27126. Nice find. Thanks for the report and demonstration.

This comment was imported from FlySpray: https://bugs.openttd.org/task/6218

@DorpsGek DorpsGek added Core flyspray This issue is imported from FlySpray (https://bugs.openttd.org/) labels Apr 7, 2018
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