#include <stdio.h>
#define MSG "Hello \
World!\n"
int main() {
  printf(MSG /* "hi!\n" */);
#ifdef __riscv
  printf("Hello RISC-V!\n");
#endif
  return 0;
}

gcc -E a.c

gcc -E a.c --verbose > /dev/null

#define max(a, b) ((a) > (b) ? (a) : (b))

riscv64-linux-gnu-gcc -E a.c  # apt-get install g++-riscv64-linux-gnu

#define _str(x) #x
#define _concat(a, b) a##b
_concat(pr, intf)(_str(RISC-V));

#include <stdio.h>
int main() { // compute 1 + 2
  int x = 1, y = 2;
  int z = x + y;
  printf("z = %d\n", z);
  return 0;
}

clang -fsyntax-only -Xclang -dump-tokens a.c

clang -fsyntax-only -Xclang -ast-dump a.c

clang -S -emit-llvm a.c

clang -S -foptimization-record-file=- a.c
clang -S -foptimization-record-file=- a.c -O1

#include <stdio.h>
int main() { // compute 1 + 2
  int x = 1, y = 2;
  volatile int z = x + y;
  printf("z = %d\n", z);
  return 0;
}

clang -S -emit-llvm -mllvm -debug-pass=Arguments a.c -O3

clang -S a.c
clang -S a.c --target=riscv64 -march=rv64g

clang -S a.c -ftime-report

clang -c a.c
clang -c a.c --target=riscv64 -march=rv64g
# alias rvclang="clang --target=riscv64 -march=rv64g"

clang a.c
riscv64-linux-gnu-gcc a.c # clang默认调用链接器的参数有点问题, 这里还是用gcc来演示
# apt-get install g++-riscv64-linux-gnu

./a.out

# 通过一些配置工作, RISC-V的可执行文件也可以在本地执行
# apt-get instal qemu-user qemu-user-binfmt
# mkdir -p /etc/qemu-binfmt
# ln -s /usr/riscv64-linux-gnu/ /etc/qemu-binfmt/riscv64
file a.out
a.out: ELF 64-bit LSB pie executable, UCB RISC-V, version 1 (SYSV)...
./a.out # 实际上是在QEMU模拟器中执行

printf(6 - 2147483648 > 6 ? "T" : "F");
printf(6 - 0x80000000 > 6 ? "T" : "F");
printf("\n");

clang -w -std=c90 -m32 a.c && ./a.out
clang -w -std=c99 -m32 a.c && ./a.out
clang -w -std=c90 -m64 a.c && ./a.out
clang -w -std=c99 -m64 a.c && ./a.out

5.2.4.2 Numerical limits

An implementation is required to document all the limits specified in this
subclause, ...

5.2.4.2.1 Sizes of integer types <limits.h>

... Their implementation-defined values shall be equal or greater in magnitude
(absolute value) to those shown, with the same sign.

... Its purpose is to promote portability, reliability, maintainability, and
efficient execution of C language programs on a variety of computing systems.

3.6 byte

NOTE 2   A byte is composed of a contiguous sequence of bits, the number of which is
implementation-defined

use of an unspecified value, or other behavior where this International Standard
provides two or more possibilities and imposes no further requirements on which is
chosen in any instance

#include <stdio.h>
void f(int x, int y) {
  printf("x = %d, y = %d\n", x, y);
}
int main() {
  int i = 1;
  f(i ++, i ++);
  return 0;
}

unspecified behavior where each implementation documents how the choice is made

assert(sizeof(long) == 4); // 在某特定32位系统上总是成立

behavior, upon use of a nonportable or erroneous program construct or of erroneous
data, for which this International Standard imposes no requirements

#include <stdio.h>
int main() {
  int a[10] = {0};
  int x = a[-2];
  printf("x = %d\n", x);
  return 0;
}

An implementation translates C source files and executes C programs in two
data-processing-system environments, which will be called the translation environment
and the execution environment in this International Standard...

char c = 0xff;
printf(c == 0xff ? "T" : "F");
printf("\n");