@Eric Postpischil

In fact, the compiler will not know.

int func1(int *);

int func2(int);

int entry() {

    int arr[10];

    {
        int _arr[10];

        func1(_arr);

        __builtin_memcpy(arr, _arr, sizeof(arr));
    }

    for (int i = 0; i < sizeof(arr) / sizeof(int); i++) {
        func2(arr[i]);
    }

    return 0;
}

        .file   "example.c"
# GNU C23 (Compiler-Explorer-Build-gcc--binutils-2.44) version 15.2.0 (x86_64-linux-gnu)
#       compiled by GNU C version 11.4.0, GMP version 6.2.1, MPFR version 4.1.0, MPC version 1.2.1, isl version isl-0.24-GMP

# GGC heuristics: --param ggc-min-expand=100 --param ggc-min-heapsize=131072
# options passed: -mtune=generic -march=x86-64 -g -g0 -Ofast -fno-asynchronous-unwind-tables
        .text
        .p2align 4
        .globl  entry
        .type   entry, @function
entry:
        pushq   %rbp  #
        pushq   %rbx  #
        subq    $104, %rsp      #,
# /app/example.c:12:         func1(_arr);
        leaq    48(%rsp), %rdi  #, tmp103
        movq    %rsp, %rbx      #, ivtmp.11
        leaq    40(%rsp), %rbp  #, _19
        call    func1   #
# /app/example.c:14:         __builtin_memcpy(arr, _arr, sizeof(arr));
        movdqa  48(%rsp), %xmm0     # MEM <unsigned char[40]> [(char * {ref-all})&_arr], MEM <unsigned char[40]> [(char * {ref-all})&_arr]
        movq    80(%rsp), %rax  # MEM <unsigned char[40]> [(char * {ref-all})&_arr], MEM <unsigned char[40]> [(char * {ref-all})&_arr]
        movaps  %xmm0, (%rsp)       # MEM <unsigned char[40]> [(char * {ref-all})&_arr], MEM <unsigned char[40]> [(char * {ref-all})&arr]
        movdqa  64(%rsp), %xmm0     # MEM <unsigned char[40]> [(char * {ref-all})&_arr], MEM <unsigned char[40]> [(char * {ref-all})&_arr]
        movq    %rax, 32(%rsp)  # MEM <unsigned char[40]> [(char * {ref-all})&_arr], MEM <unsigned char[40]> [(char * {ref-all})&arr]
        movaps  %xmm0, 16(%rsp)     # MEM <unsigned char[40]> [(char * {ref-all})&_arr], MEM <unsigned char[40]> [(char * {ref-all})&arr]
        .p2align 4
        .p2align 3
.L2:
# /app/example.c:18:         func2(arr[i]);
        movl    (%rbx), %edi    # MEM[(int *)_17], MEM[(int *)_17]
# /app/example.c:17:     for (int i = 0; i < sizeof(arr) / sizeof(int); i++) {
        addq    $4, %rbx        #, ivtmp.11
# /app/example.c:18:         func2(arr[i]);
        call    func2   #
# /app/example.c:17:     for (int i = 0; i < sizeof(arr) / sizeof(int); i++) {
        cmpq    %rbp, %rbx      # _19, ivtmp.11
        jne     .L2       #,
# /app/example.c:22: }
        addq    $104, %rsp      #,
        xorl    %eax, %eax      #
        popq    %rbx    #
        popq    %rbp    #
        ret     
        .size   entry, .-entry
        .ident  "GCC: (Compiler-Explorer-Build-gcc--binutils-2.44) 15.2.0"
        .section        .note.GNU-stack,"",@progbits

The compiler will still allocate space to save it.

The best optimization method is to dereference each time func2(arr[i]); is called.

The compiler does indeed have the ability to do it.

int func1(int *);

int func2(int);

__attribute__((malloc)) int *func3();

int entry() {

    int *arr = func3();

    for (int i = 0; i < 10; i++) {
        func2(arr[i]);
    }

    return 0;
}

        .file   "example.c"
# GNU C23 (Compiler-Explorer-Build-gcc--binutils-2.44) version 15.2.0 (x86_64-linux-gnu)
#       compiled by GNU C version 11.4.0, GMP version 6.2.1, MPFR version 4.1.0, MPC version 1.2.1, isl version isl-0.24-GMP

# GGC heuristics: --param ggc-min-expand=100 --param ggc-min-heapsize=131072
# options passed: -mtune=generic -march=x86-64 -g -g0 -Ofast -fno-asynchronous-unwind-tables
        .text
        .p2align 4
        .globl  entry
        .type   entry, @function
entry:
        pushq   %rbp  #
        pushq   %rbx  #
        subq    $8, %rsp        #,
# /app/example.c:9:     int *arr = func3();
        call    func3   #
        movq    %rax, %rbx      # ivtmp.10, ivtmp.10
        leaq    40(%rax), %rbp  #, _20
        .p2align 4
        .p2align 3
.L2:
# /app/example.c:12:         func2(arr[i]);
        movl    (%rbx), %edi    # MEM[(int *)_18], MEM[(int *)_18]
# /app/example.c:11:     for (int i = 0; i < 10; i++) {
        addq    $4, %rbx        #, ivtmp.10
# /app/example.c:12:         func2(arr[i]);
        call    func2   #
# /app/example.c:11:     for (int i = 0; i < 10; i++) {
        cmpq    %rbp, %rbx      # _20, ivtmp.10
        jne     .L2       #,
# /app/example.c:16: }
        addq    $8, %rsp        #,
        xorl    %eax, %eax      #
        popq    %rbx    #
        popq    %rbp    #
        ret     
        .size   entry, .-entry
        .ident  "GCC: (Compiler-Explorer-Build-gcc--binutils-2.44) 15.2.0"
        .section        .note.GNU-stack,"",@progbits