Down into the depths we go

#include <stdio.h>
#include <string.h>
#include <stdlib.h>

int main (void){
	int n = 10000;
	char* s1 = (char *) malloc(sizeof(char) * n);
	char* s2 = (char *) malloc(sizeof(char) * n);
	strcpy(s1,"Hello World!");
	strcpy(s2,strcat(s1,s1+1));
	printf("%s",s2);
	return 0;
}

Original version

    .section    __TEXT,__text,regular,pure_instructions
    .globl  _main
    .align  4, 0x90
_main:                                  ## @main
    .cfi_startproc
## BB#0:
    pushq   %rbp
Ltmp2:
    .cfi_def_cfa_offset 16
Ltmp3:
    .cfi_offset %rbp, -16
    movq    %rsp, %rbp
Ltmp4:
    .cfi_def_cfa_register %rbp
    subq    $64, %rsp
    movl    $0, -4(%rbp)
    movl    $10000, -8(%rbp)        ## imm = 0x2710
    movslq  -8(%rbp), %rax
    shlq    $0, %rax
    movq    %rax, %rdi
    callq   _malloc
    movq    %rax, -16(%rbp)
    movslq  -8(%rbp), %rax
    shlq    $0, %rax
    movq    %rax, %rdi
    callq   _malloc
    leaq    L_.str(%rip), %rsi
    movabsq $-1, %rdx
    movq    %rax, -24(%rbp)
    movq    -16(%rbp), %rdi
    callq   ___strcpy_chk
    movabsq $-1, %rdx
    movq    -24(%rbp), %rdi
    movq    -16(%rbp), %rsi
    movq    -16(%rbp), %rcx
    addq    $1, %rcx
    movq    %rdi, -32(%rbp)         ## 8-byte Spill
    movq    %rsi, %rdi
    movq    %rcx, %rsi
    movq    %rax, -40(%rbp)         ## 8-byte Spill
    callq   ___strcat_chk
    movabsq $-1, %rdx
    movq    -32(%rbp), %rdi         ## 8-byte Reload
    movq    %rax, %rsi
    callq   ___strcpy_chk
    leaq    L_.str1(%rip), %rdi
    movq    -24(%rbp), %rsi
    movq    %rax, -48(%rbp)         ## 8-byte Spill
    movb    $0, %al
    callq   _printf
    movl    $0, %r8d
    movl    %eax, -52(%rbp)         ## 4-byte Spill
    movl    %r8d, %eax
    addq    $64, %rsp
    popq    %rbp
    ret
    .cfi_endproc

    .section    __TEXT,__cstring,cstring_literals
L_.str:                                 ## @.str
    .asciz  "Hello World!"

L_.str1:                                ## @.str1
    .asciz  "%s"


.subsections_via_symbols

Nathan's Version

    .section    __TEXT,__text,regular,pure_instructions
    .globl  _main
    .align  4, 0x90
_main:                                  ## @main
    .cfi_startproc
## BB#0:
    pushq   %rbp
Ltmp2:
    .cfi_def_cfa_offset 16
Ltmp3:
    .cfi_offset %rbp, -16
    movq    %rsp, %rbp
Ltmp4:
    .cfi_def_cfa_register %rbp
    subq    $64, %rsp
    movl    $0, -4(%rbp)
    movl    $10000, -8(%rbp)        ## imm = 0x2710
    movslq  -8(%rbp), %rax
    shlq    $0, %rax
    movq    %rax, %rdi
    callq   _malloc
    movq    %rax, -16(%rbp)
    movslq  -8(%rbp), %rax
    shlq    $0, %rax
    movq    %rax, %rdi
    callq   _malloc
    leaq    L_.str(%rip), %rsi
    movabsq $-1, %rdx
    movq    %rax, -24(%rbp)
    movq    -16(%rbp), %rdi
    callq   ___strcpy_chk
    movq    -24(%rbp), %rdi
    movq    -16(%rbp), %rdx
    movq    %rdi, -32(%rbp)         ## 8-byte Spill
    movq    %rdx, %rdi
    movq    %rax, -40(%rbp)         ## 8-byte Spill
    callq   _strdup
    movabsq $-1, %rdx
    movq    -16(%rbp), %rsi
    addq    $1, %rsi
    movq    %rax, %rdi
    callq   ___strcat_chk
    movabsq $-1, %rdx
    movq    -32(%rbp), %rdi         ## 8-byte Reload
    movq    %rax, %rsi
    callq   ___strcpy_chk
    leaq    L_.str1(%rip), %rdi
    movq    -24(%rbp), %rsi
    movq    %rax, -48(%rbp)         ## 8-byte Spill
    movb    $0, %al
    callq   _printf
    movl    $0, %ecx
    movl    %eax, -52(%rbp)         ## 4-byte Spill
    movl    %ecx, %eax
    addq    $64, %rsp
    popq    %rbp
    ret
    .cfi_endproc

    .section    __TEXT,__cstring,cstring_literals
L_.str:                                 ## @.str
    .asciz  "Hello World!"

L_.str1:                                ## @.str1
    .asciz  "%s"


.subsections_via_symbols