README.md

$ time ./a.out
4./a.out  18.75s user 0.01s system 99% cpu 18.879 total

$ time ./b.out
4./b.out  23.44s user 0.02s system 99% cpu 23.550 total

$ gcc --version
Configured with: --prefix=/Library/Developer/CommandLineTools/usr --with-gxx-include-dir=/Library/Developer/CommandLineTools/SDKs/MacOSX.sdk/usr/include/c++/4.2.1
Apple clang version 11.0.0 (clang-1100.0.33.8)
Target: x86_64-apple-darwin19.0.0
Thread model: posix
InstalledDir: /Library/Developer/CommandLineTools/usr/bin

a.s, b.sはそれぞれ関数fに関する抜粋。

a.c

#include <stdio.h>
#define LOOP 1000000000

int main(int argc, char *argv[]){
  unsigned int i;
  int x;

  for (i = 0; i < LOOP; i++) {
    x = f(1, 1, 1, 1);
  }
  for (i = 0; i < LOOP; i++) {
    x = f(1, 1, 1, 0);
  }
  for (i = 0; i < LOOP; i++) {
    x = f(1, 1, 0, 0);
  }
  for (i = 0; i < LOOP; i++) {
    x = f(1, 0, 0, 0);
  }
  for (i = 0; i < LOOP; i++) {
    x = f(0, 0, 0, 0);
  }
  printf("%d", x);

  return 0;
}

int f(int a, int b, int c, int d) {
  int m;

  if (a == 1) {
    if (b == 1) {
      if (c == 1) {
        if (d == 1) {
          m = g();
        } else {
          m = 1;
        }
      } else {
        m = 2;
      }
    } else {
      m = 3;
    }
  } else {
    m = 4;
  }

  return m;
}

int g() {
  return 0;
}

a.s

	.globl	_f                      ## -- Begin function f
	.p2align	4, 0x90
_f:                                     ## @f
Lfunc_begin1:
	.loc	1 15 0                  ## a.c:15:0
	.cfi_startproc
## %bb.0:
	pushq	%rbp
	.cfi_def_cfa_offset 16
	.cfi_offset %rbp, -16
	movq	%rsp, %rbp
	.cfi_def_cfa_register %rbp
	subq	$32, %rsp
	movl	%edi, -4(%rbp)
	movl	%esi, -8(%rbp)
	movl	%edx, -12(%rbp)
	movl	%ecx, -16(%rbp)
Ltmp7:
	.loc	1 18 9 prologue_end     ## a.c:18:9
	cmpl	$1, -4(%rbp)
Ltmp8:
	.loc	1 18 7 is_stmt 0        ## a.c:18:7
	jne	LBB1_11
## %bb.1:
Ltmp9:
	.loc	1 19 11 is_stmt 1       ## a.c:19:11
	cmpl	$1, -8(%rbp)
Ltmp10:
	.loc	1 19 9 is_stmt 0        ## a.c:19:9
	jne	LBB1_9
## %bb.2:
Ltmp11:
	.loc	1 20 13 is_stmt 1       ## a.c:20:13
	cmpl	$1, -12(%rbp)
Ltmp12:
	.loc	1 20 11 is_stmt 0       ## a.c:20:11
	jne	LBB1_7
## %bb.3:
Ltmp13:
	.loc	1 21 15 is_stmt 1       ## a.c:21:15
	cmpl	$1, -16(%rbp)
Ltmp14:
	.loc	1 21 13 is_stmt 0       ## a.c:21:13
	jne	LBB1_5
## %bb.4:
Ltmp15:
	.loc	1 22 15 is_stmt 1       ## a.c:22:15
	callq	_g
	.loc	1 22 13 is_stmt 0       ## a.c:22:13
	movl	%eax, -20(%rbp)
	.loc	1 23 9 is_stmt 1        ## a.c:23:9
	jmp	LBB1_6
Ltmp16:
LBB1_5:
	.loc	1 24 13                 ## a.c:24:13
	movl	$1, -20(%rbp)
Ltmp17:
LBB1_6:
	.loc	1 26 7                  ## a.c:26:7
	jmp	LBB1_8
Ltmp18:
LBB1_7:
	.loc	1 27 11                 ## a.c:27:11
	movl	$2, -20(%rbp)
Ltmp19:
LBB1_8:
	.loc	1 29 5                  ## a.c:29:5
	jmp	LBB1_10
Ltmp20:
LBB1_9:
	.loc	1 30 9                  ## a.c:30:9
	movl	$3, -20(%rbp)
Ltmp21:
LBB1_10:
	.loc	1 32 3                  ## a.c:32:3
	jmp	LBB1_12
Ltmp22:
LBB1_11:
	.loc	1 33 7                  ## a.c:33:7
	movl	$4, -20(%rbp)
Ltmp23:
LBB1_12:
	.loc	1 36 10                 ## a.c:36:10
	movl	-20(%rbp), %eax
	.loc	1 36 3 is_stmt 0        ## a.c:36:3
	addq	$32, %rsp
	popq	%rbp
	retq
Ltmp24:
Lfunc_end1:
	.cfi_endproc
                                        ## -- End function

b.c

#include <stdio.h>
#define LOOP 1000000000

int main(int argc, char *argv[]){
  unsigned int i;
  int x;

  for (i = 0; i < LOOP; i++) {
    x = f(1, 1, 1, 1);
  }
  for (i = 0; i < LOOP; i++) {
    x = f(1, 1, 1, 0);
  }
  for (i = 0; i < LOOP; i++) {
    x = f(1, 1, 0, 0);
  }
  for (i = 0; i < LOOP; i++) {
    x = f(1, 0, 0, 0);
  }
  for (i = 0; i < LOOP; i++) {
    x = f(0, 0, 0, 0);
  }
  printf("%d", x);

  return 0;
}

int f(int a, int b, int c, int d) {
  int m;

  if (a != 1) {
    m = 4;
  } else if (b != 1) {
    m = 3;
  } else if (c != 1) {
    m = 2;
  } else if (d != 1) {
    m = 1;
  } else {
    m = g();
  }

  return m;
}

int g() {
  return 0;
}

b.s

	.globl	_f                      ## -- Begin function f
	.p2align	4, 0x90
_f:                                     ## @f
Lfunc_begin1:
	.loc	1 15 0                  ## b.c:15:0
	.cfi_startproc
## %bb.0:
	pushq	%rbp
	.cfi_def_cfa_offset 16
	.cfi_offset %rbp, -16
	movq	%rsp, %rbp
	.cfi_def_cfa_register %rbp
	subq	$32, %rsp
	movl	%edi, -4(%rbp)
	movl	%esi, -8(%rbp)
	movl	%edx, -12(%rbp)
	movl	%ecx, -16(%rbp)
Ltmp7:
	.loc	1 18 9 prologue_end     ## b.c:18:9
	cmpl	$1, -4(%rbp)
Ltmp8:
	.loc	1 18 7 is_stmt 0        ## b.c:18:7
	je	LBB1_2
## %bb.1:
Ltmp9:
	.loc	1 19 7 is_stmt 1        ## b.c:19:7
	movl	$4, -20(%rbp)
	.loc	1 20 3                  ## b.c:20:3
	jmp	LBB1_12
Ltmp10:
LBB1_2:
	.loc	1 20 16 is_stmt 0       ## b.c:20:16
	cmpl	$1, -8(%rbp)
Ltmp11:
	.loc	1 20 14                 ## b.c:20:14
	je	LBB1_4
## %bb.3:
Ltmp12:
	.loc	1 21 7 is_stmt 1        ## b.c:21:7
	movl	$3, -20(%rbp)
	.loc	1 22 3                  ## b.c:22:3
	jmp	LBB1_11
Ltmp13:
LBB1_4:
	.loc	1 22 16 is_stmt 0       ## b.c:22:16
	cmpl	$1, -12(%rbp)
Ltmp14:
	.loc	1 22 14                 ## b.c:22:14
	je	LBB1_6
## %bb.5:
Ltmp15:
	.loc	1 23 7 is_stmt 1        ## b.c:23:7
	movl	$2, -20(%rbp)
	.loc	1 24 3                  ## b.c:24:3
	jmp	LBB1_10
Ltmp16:
LBB1_6:
	.loc	1 24 16 is_stmt 0       ## b.c:24:16
	cmpl	$1, -16(%rbp)
Ltmp17:
	.loc	1 24 14                 ## b.c:24:14
	je	LBB1_8
## %bb.7:
Ltmp18:
	.loc	1 25 7 is_stmt 1        ## b.c:25:7
	movl	$1, -20(%rbp)
	.loc	1 26 3                  ## b.c:26:3
	jmp	LBB1_9
Ltmp19:
LBB1_8:
	.loc	1 27 9                  ## b.c:27:9
	callq	_g
	.loc	1 27 7 is_stmt 0        ## b.c:27:7
	movl	%eax, -20(%rbp)
Ltmp20:
LBB1_9:
	.loc	1 0 7                   ## b.c:0:7
	jmp	LBB1_10
LBB1_10:
	jmp	LBB1_11
LBB1_11:
	jmp	LBB1_12
LBB1_12:
	.loc	1 30 10 is_stmt 1       ## b.c:30:10
	movl	-20(%rbp), %eax
	.loc	1 30 3 is_stmt 0        ## b.c:30:3
	addq	$32, %rsp
	popq	%rbp
	retq
Ltmp21:
Lfunc_end1:
	.cfi_endproc
                                        ## -- End function