Which part of the register is used?

clang34-int.s

	.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
	movl	$0, -4(%rbp)
	movl	$15, -8(%rbp)
	movl	$20, -12(%rbp)
	movl	-8(%rbp), %eax
	imull	-12(%rbp), %eax
	movl	%eax, -16(%rbp)
	movl	-16(%rbp), %eax
	popq	%rbp
	ret
	.cfi_endproc


.subsections_via_symbols

clang34-sizet.s

	.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
	movl	$0, -4(%rbp)
	movq	$15, -16(%rbp)
	movq	$20, -24(%rbp)
	movq	-16(%rbp), %rax
	imulq	-24(%rbp), %rax
	movq	%rax, -32(%rbp)
	movq	-32(%rbp), %rax
	movl	%eax, %ecx
	movl	%ecx, %eax
	popq	%rbp
	ret
	.cfi_endproc


.subsections_via_symbols

register.c

/**
 * compile to asm and view with:
 *
 * cc -DTYPE=size_t register.c -S -o register.s && vim register.s
 * cc -DTYPE=int register.c -S -o register.s && vim register.s
 */
#include <stdio.h>
#include <stdint.h>

typedef TYPE int_t;

int main() {
    int_t i = 15;
    int_t m = 20;
    int_t a = i * m;

    return a;
}