When I allocate the key on the heap, the memset is carried (heavily optimized and inlined). When I allocate key on the stack, it disappears. Using gcc -03:

    #include <string.h>

    void doSecure(void)  
    {  
        /*char key[32];*/  
        char *key = (char*) malloc(sizeof(char)*32);

        memset(key,sizeof(char),32);  
    }

    int main(void)  
    {  
        doSecure();

        return 0;  
    }

    -- key on stack

    main:  
    .LFB13:  
        .cfi_startproc  
        xorl	%eax, %eax  
        ret  
        .cfi_endproc

    -- key on heap

    main:  
    .LFB13:  
        .cfi_startproc  
        subq	$8, %rsp  
        .cfi_def_cfa_offset 16  
        movl	$32, %edi  
        call	malloc  
        movabsq	$72340172838076673, %rdx  
        movq	%rdx, (%rax)  
        movq	%rdx, 8(%rax)  
        movq	%rdx, 16(%rax)  
        movq	%rdx, 24(%rax)  
        xorl	%eax, %eax  
        addq	$8, %rsp  
        .cfi_def_cfa_offset 8  
        ret  
        .cfi_endproc