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C front-end: fix processing of alignment and packing attributes #8454

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Packing needs to be evaluated as part of the definition of the type while alignment applies also when using a type. The position of the attribute also requires extra care as some positions are accepted by GCC (and warned about by Clang) while actually being ignored.

Fixes: #8443

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codecov bot commented Sep 13, 2024

Codecov Report

Attention: Patch coverage is 92.85714% with 3 lines in your changes missing coverage. Please review.

Project coverage is 80.37%. Comparing base (50be009) to head (0367e2c).

Files with missing lines Patch % Lines
src/ansi-c/c_typecheck_type.cpp 91.66% 2 Missing ⚠️
src/ansi-c/parser.y 94.44% 1 Missing ⚠️
Additional details and impacted files
@@           Coverage Diff            @@
##           develop    #8454   +/-   ##
========================================
  Coverage    80.37%   80.37%           
========================================
  Files         1686     1686           
  Lines       206888   206928   +40     
  Branches        73       78    +5     
========================================
+ Hits        166288   166325   +37     
- Misses       40600    40603    +3     

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power(2, align_int->floorPow2()) != *align_int)
{
throw errort().with_location(type.source_location())
<< "alignment is not a positive power of 2";
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Is that a requirement in the standard?

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C11 6.2.8 "Alignment of objects" includes the following sentence: "Every valid alignment value shall be a nonnegative integral power of two."

Packing needs to be evaluated as part of the definition of the type
while alignment applies also when using a type. The position of the
attribute also requires extra care as some positions are accepted by GCC
(and warned about by Clang) while actually being ignored.

Fixes: diffblue#8443
@tautschnig tautschnig force-pushed the fix-8443-attributes branch from 5eea045 to 0367e2c Compare May 28, 2025 11:16
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__attribute__((packed)) incorrectly handled when used in member of a struct
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