Difference between revisions of "Packed"
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In the example above <syntaxhighlight lang="pascal" enclose="none">signedQword</syntaxhighlight> as well <syntaxhighlight lang="pascal" enclose="none">signedQwordPacked</syntaxhighlight> have <syntaxhighlight lang="pascal" enclose="none">signum</syntaxhighlight> located at an offset of <syntaxhighlight lang="pascal" enclose="none">8</syntaxhighlight>. | In the example above <syntaxhighlight lang="pascal" enclose="none">signedQword</syntaxhighlight> as well <syntaxhighlight lang="pascal" enclose="none">signedQwordPacked</syntaxhighlight> have <syntaxhighlight lang="pascal" enclose="none">signum</syntaxhighlight> located at an offset of <syntaxhighlight lang="pascal" enclose="none">8</syntaxhighlight>. | ||
That is OK. | That is OK. | ||
| − | If for any reason <syntaxhighlight lang="pascal" enclose="none">value</syntaxhighlight> and <syntaxhighlight lang="pascal" enclose="none">signum</syntaxhighlight> were listed in reverse order, <syntaxhighlight lang="pascal" enclose="none">signum</syntaxhighlight> would appear in both | + | If for any reason <syntaxhighlight lang="pascal" enclose="none">value</syntaxhighlight> and <syntaxhighlight lang="pascal" enclose="none">signum</syntaxhighlight> were listed in reverse order, <syntaxhighlight lang="pascal" enclose="none">signum</syntaxhighlight> would appear in both versions at an offset of <syntaxhighlight lang="pascal" enclose="none">0</syntaxhighlight>, but in the <syntaxhighlight lang="pascal" enclose="none">packed</syntaxhighlight> version <syntaxhighlight lang="pascal" enclose="none">value</syntaxhighlight> has an offset of <syntaxhighlight lang="pascal" enclose="none">1</syntaxhighlight> Byte. |
This might cause a potentially slower access to the <syntaxhighlight lang="pascal" enclose="none">value</syntaxhighlight> field. | This might cause a potentially slower access to the <syntaxhighlight lang="pascal" enclose="none">value</syntaxhighlight> field. | ||
Revision as of 21:09, 20 May 2018
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The reserved word packed tells the compiler to use as little memory as possible for a particular complex data type.
Without specifying packed, the compiler may insert extra unused bytes between members in order to align the data on full word boundaries for faster access by the CPU.
1program packedDemo(input, output, stderr);
2
3type
4 signedQword = record
5 value: qword;
6 signum: -1..1;
7 end;
8
9 signedQwordPacked = packed record
10 value: qword;
11 signum: -1..1;
12 end;
13
14begin
15 writeLn(' signedQword: ', sizeOf(signedQword), 'B');
16 writeLn('signedQwordPacked: ', sizeOf(signedQwordPacked), 'B');
17end.
So packed sacrifices some speed while reducing the memory used.
circumvention
Smart ordering of record elements can mitigate the issue.
In the example above signedQword as well signedQwordPacked have signum located at an offset of 8.
That is OK.
If for any reason value and signum were listed in reverse order, signum would appear in both versions at an offset of 0, but in the packed version value has an offset of 1 Byte.
This might cause a potentially slower access to the value field.
