gcc and clang vs. https://randomascii.wordpress.com/2016/07/17/zeroing-memory-is-hard-vc-2015-arrays/

gistfile1.txt

# - Host is OS X 10.11.5 (Darwin 15.5.0)
# - clang is the version shipped with XCode 7.3.1: "Apple LLVM version 7.3.0 (clang-703.0.31)"
# - gcc-6.1.0 was compiled from source

% cat z.c
extern void do_something(char *);

main() {
	char buffer[32] = { 0 };
	do_something(buffer);
}

% gcc-6.1.0 -m64 -march=x86-64 -mtune=generic -O0 -S -o - z.c
[snip]
        movq    $0, -32(%rbp)
        movq    $0, -24(%rbp)
        movq    $0, -16(%rbp)
        movq    $0, -8(%rbp)
[snip]

% gcc-6.1.0 -m64 -march=x86-64 -mtune=generic -O1 -S -o - z.c
[snip]
        movq    $0, (%rsp)
        movq    $0, 8(%rsp)
        movq    $0, 16(%rsp)
        movq    $0, 24(%rsp)
[snip]

# NOTE: -O2 and -O3 generate the same assembly code as -O1

% gcc-6.1.0 -m64 -march=x86-64 -mtune=generic -Os -S -o - z.c
[snip]
        xorl    %eax, %eax
        movl    $8, %ecx
        movq    %rsp, %rdi
        rep stosl
[snip]

% clang -m64 -march=x86-64 -mtune=generic -fno-stack-protector -O0 -S -o - z.c
[snip]
        xorl    %esi, %esi
        movl    $32, %ecx
        movl    %ecx, %edx
        movq    %rax, %rdi
        movq    %rax, -40(%rbp)         ## 8-byte Spill
        callq   _memset
[snip]

% clang -m64 -march=x86-64 -mtune=generic -fno-stack-protector -O1 -S -o - z.c
[snip]
        xorps   %xmm0, %xmm0
        movaps  %xmm0, -16(%rbp)
        movaps  %xmm0, -32(%rbp)
[snip]

# NOTE: -O2, -O3, and -Os generate the same assembly code as -O1