Bare arrays as function args in C and C++ always decay to pointers, just like in several other contexts. Arrays inside structs or unions don’t, and are passed by value. This is why ABIs need to care about how they’re passed, even though it doesn’t happen in C for bare arrays. As Keith Thomson points … Read more
The trick is that you tell them somehow else. For printf you have to supply a format string which even contains type information (which might be incorrect though). The way to supply this information is mainly user-contract and often error-prone. As for calling conventions: Usually the arguments are pushed onto the stack from left to … Read more
instead of put the first 6 arguments in registers just to move them onto the stack in the function prologue? I was looking at some code that gcc generated and that’s what it always did. Then you forgot to enable optimization. gcc -O0 spills everything to memory so you can modify them with a debugger … Read more
The problem is with your stack usage. First, the ABI docs mandate rsp be 16 byte aligned before a call. Since a call will push an 8 byte return address on the stack, you need to adjust rsp by a multiple of 16 plus 8 to get back to 16-byte alignment. The 16 * n … Read more
None; no copies are done. The address of the caller’s Data return value is actually passed as a hidden argument to the function, and the createData function simply writes into the caller’s stack frame. This is known as the named return value optimisation. Also see the c++ faq on this topic. commercial-grade C++ compilers implement … Read more
Here is what the gcc manual has to say about it: -mpush-args -mno-push-args Use PUSH operations to store outgoing parameters. This method is shorter and usually equally fast as method using SUB/MOV operations and is enabled by default. In some cases disabling it may improve performance because of improved scheduling and reduced dependencies. -maccumulate-outgoing-args If … Read more
The ABI specification is defined here. A newer version is available here. I assume the reader is accustomed to the terminology of the document and that they can classify the primitive types. If the object size is larger than two eight-bytes, it is passed in memory: struct foo { unsigned long long a; unsigned long … Read more
Is this actually an ABI requirement, or maybe it’s just some pessimization in certain scenarios? One example is System V Application Binary Interface AMD64 Architecture Processor Supplement. This ABI is for 64-bit x86-compatible CPUs (Linux x86_64 architecure). It is followed on Solaris, Linux, FreeBSD, macOS, Windows Subsystem for Linux: If a C++ object has either … Read more