core/com.nokia.carbide.cpp.compiler.doc.user/html/pragmas_x86/p_stack_alignment.htm
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<h3>stack_alignment</h3>
<p> Recommends the minimum stack alignment size based on a power of 2.</p>
<h5>Syntax</h5>
<p class="code">#pragma stack_alignment( power )</p>
<h5>Parameters</h5>
<p class="code">power</p>
<blockquote>
<p>Stack alignment value based on a power of 2.</p>
</blockquote>
<h5>Targets</h5>
Intel x86<br />
<h5>Remarks</h5>
<p>By default, the global stack alignment is 4 as per the Win32 ABI. When doubles are accessed in a function, the alignment is bumped to 8 for that function. Spills of SSE registers (e.g. in autovectorized code or code using SSE intrinsics) bump alignment to 16 bytes.</p>
<p>Finally, the GCC-style <span class="code">__attribute__((aligned(xxx)))</span> on a local variable declaration can be used to align specific variables, which in turn aligns the stack while use of <span class="code">#pragma disablopts off</span> turns off stack alignment.</p>
<p>This pragma recommends a minimum alignment. When one of the above situations occurs, the actual alignment may be greater.<br />
In some functions, in order to guarantee alignment, the EBX register is used as a surrogate local base pointer. This happens in functions which have indeterminate control flow (i.e., jump through a pointer), which are involved in exception handling, or which use <span class="code">alloca()</span>. Use of EBX to access locals is not possible if Microsoft C++ exception handling is used in the function, so alignment is disabled for such functions. You can use <span class="code">#pragma warn_stack_alignment</span> to check for such cases.<br />
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