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    <p>Yes most compilers do push on stack, or copy to stack value-passed structures (and <code>class</code>es) . It is generally required by the <a href="http://en.wikipedia.org/wiki/Application_binary_interface" rel="nofollow">ABI</a> (application binary interface) specification relative to the compiler &amp; processor &amp; operating system.</p> <p>See e.g. <a href="http://en.wikipedia.org/wiki/X86_calling_conventions" rel="nofollow">X86 calling conventions</a> &amp; <a href="http://www.yumpu.com/en/document/view/5100681/system-v-application-binary-interface-x86-64-linux" rel="nofollow">System V ABI x86-64</a> for details (at least for Linux, x86-64).</p> <p>Practically speaking, large structures are on the stack, passing (silently) a pointer to them thru registers.</p> <p><sup>The ABI defines if these structures are in the caller or callee call frame...</sup></p> <p>For two-word sized <code>struct</code>-s the Linux x86-64 ABI often passes them (both as argument &amp; as result) thru a pair of registers.</p> <p>With <a href="http://gcc.gnu.org/" rel="nofollow">GCC</a>, try compiling using <code>gcc -O -S -fverbose-asm foo.c</code> to get the assembly code <code>foo.s</code>; you could use also <a href="http://gcc-melt.org/#probe_id" rel="nofollow">GCC MELT probe</a> or <code>gcc -fdump-tree-all</code> to understand the internal (Gimple) representations.</p> <p>Notice that some quite complex <code>class</code> in C++ may have a small value size, because internally a lot of pointers are involved. For instance, on Linux/AMD64 <code>sizeof(std::string)</code> is just one (8 byte) word (containing a pointer to some complex stuff), which is probably passed by register. Likewise many containers of the C++ standard library have a small value size (most of the real data being indirectly accessed thru pointers). Details are obviously implementation specific.</p>
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    1. POAre pass-by-value structs pushed onto the stack?
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    1. COdo caller do that - (such pushlike memcopy) ?
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    2. CODetails depend upon the ABI.
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      1. USBasile Starynkevitch
 

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