LOCAL_DECL_ALIGNMENT Macro documentation 1172 @defmac LOCAL_DECL_ALIGNMENT (@var{decl}) 1173 If defined, a C expression to compute the alignment for a local 1174 variable @var{decl}. 1175 1176 If this macro is not defined, then 1177 @code{LOCAL_ALIGNMENT (TREE_TYPE (@var{decl}), DECL_ALIGN (@var{decl}))} 1178 is used. 1179 1180 One use of this macro is to increase alignment of medium-size data to 1181 make it all fit in fewer cache lines. 1182 1183 If the value of this macro has a type, it should be an unsigned type. 1184 @end defmac This macro not only increases alignment but also decreases(-m32 -mpreferred-stack-boundary=2) depending on condition.
The documentation is correct, the x86 implementation is not. Note the documentation says "One use of this macro...", so clearly it's documentation can be improved by explicitely constraining the macro to increase alignment.
There is now TARGET_LOWER_LOCAL_DECL_ALIGNMENT which was added by r11-2259-g 0a9d711df36b42 . We should provide examples of when one should be used vs the other. And maybe even better describe why there are two target hooks that seems like does the same thing now. I am going to try to this for GCC 14 with the other target macro/hooks cleanups I am doing.