我有一个简单的C程序。例如，假设我有一个int和一个长度为20的char数组。我总共需要24个字节。
int main()
{
char buffer[20];
int x = 0;
buffer[0] = 'a';
buffer[19] = 'a';
}
堆栈需要对齐到16个字节的边界，因此我假设编译器将保留32个字节。但是，当我使用gcc x86-64编译这样的程序并读取输出程序集时，编译器会保留64个字节。
..\gcc -S -o main.s main.c
给我:
.file "main.c"
.def __main; .scl 2; .type 32; .endef
.text
.globl main
.def main; .scl 2; .type 32; .endef
.seh_proc main
main:
pushq %rbp # RBP is pushed, so no need to reserve more for it
.seh_pushreg %rbp
movq %rsp, %rbp
.seh_setframe %rbp, 0
subq $64, %rsp # Reserving the 64 bytes
.seh_stackalloc 64
.seh_endprologue
call __main
movl $0, -4(%rbp) # Using the first 4 bytes to store the int
movb $97, -32(%rbp) # Using from RBP-32
movb $97, -13(%rbp) # to RBP-13 to store the char array
movl $0, %eax
addq $64, %rsp # Restoring the stack with the last 32 bytes unused
popq %rbp
ret
.seh_endproc
.ident "GCC: (x86_64-posix-seh-rev0, Built by MinGW-W64 project) 5.2.0"
这是为什么？当我编写汇编程序时，我总是只保留所需的最小内存，没有任何问题。这是编译器的局限性吗，它无法评估所需的内存，还是有原因呢？
这是gcc -v
Using built-in specs.
COLLECT_GCC=gcc
COLLECT_LTO_WRAPPER=D:/Mingw64/bin/../libexec/gcc/x86_64-w64-mingw32/5.2.0/lto-wrapper.exe
Target: x86_64-w64-mingw32
Configured with: ../../../src/gcc-5.2.0/configure --host=x86_64-w64-mingw32 --build=x86_64-w64-mingw32 --target=x86_64-w64-mingw32 --prefix=/mingw64 --with-sysroot=/c/mingw520/x86_64-520-posix-seh-rt_v4-rev0/mingw64 --with-gxx-include-dir=/mingw64/x86_64-w64-mingw32/include/c++ --enable-shared --enable-static --disable-multilib --enable-languages=c,c++,fortran,objc,obj-c++,lto --enable-libstdcxx-time=yes --enable-threads=posix --enable-libgomp --enable-libatomic --enable-lto --enable-graphite --enable-checking=release --enable-fully-dynamic-string --enable-version-specific-runtime-libs --disable-isl-version-check --disable-libstdcxx-pch --disable-libstdcxx-debug --enable-bootstrap --disable-rpath --disable-win32-registry --disable-nls --disable-werror --disable-symvers --with-gnu-as --with-gnu-ld --with-arch=nocona --with-tune=core2 --with-libiconv --with-system-zlib --with-gmp=/c/mingw520/prerequisites/x86_64-w64-mingw32-static --with-mpfr=/c/mingw520/prerequisites/x86_64-w64-mingw32-static --with-mpc=/c/mingw520/prerequisites/x86_64-w64-mingw32-static --with-isl=/c/mingw520/prerequisites/x86_64-w64-mingw32-static --with-pkgversion='x86_64-posix-seh-rev0, Built by MinGW-W64 project' --with-bugurl=http://sourceforge.net/projects/mingw-w64 CFLAGS='-O2 -pipe -I/c/mingw520/x86_64-520-posix-seh-rt_v4-rev0/mingw64/opt/include -I/c/mingw520/prerequisites/x86_64-zlib-static/include -I/c/mingw520/prerequisites/x86_64-w64-mingw32-static/include' CXXFLAGS='-O2 -pipe -I/c/mingw520/x86_64-520-posix-seh-rt_v4-rev0/mingw64/opt/include -I/c/mingw520/prerequisites/x86_64-zlib-static/include -I/c/mingw520/prerequisites/x86_64-w64-mingw32-static/include' CPPFLAGS= LDFLAGS='-pipe -L/c/mingw520/x86_64-520-posix-seh-rt_v4-rev0/mingw64/opt/lib -L/c/mingw520/prerequisites/x86_64-zlib-static/lib -L/c/mingw520/prerequisites/x86_64-w64-mingw32-static/lib '
Thread model: posix
gcc version 5.2.0 (x86_64-posix-seh-rev0, Built by MinGW-W64 project)
最佳答案
编译器的确可以为自己保留更多的内存。
Gcc有一个标记-mpreferred-stack-boundary，用于设置它将保持的对齐方式。根据the documentation，默认值为4，这将产生16字节对齐，这是SSE指令所需的。
作为VermillionAzure noted in a comment，您应该提供gcc版本和编译时选项(使用gcc -v显示这些内容)。
关于c - 为什么编译器在堆栈中分配的空间超出了需要？，我们在Stack Overflow上找到一个类似的问题：https://stackoverflow.com/questions/37770751/