C benchmark demonstrating bizarre performance behavior of Intel CPUs

Loop benchmark

const unsigned N = 400 * 1000 * 1000;

volatile unsigned long long counter = 0;

// Don't inline the benchmarking code into main
void __attribute__((noinline)) tightloop();
void __attribute__((noinline)) loop_with_extra_call();

void tightloop() {
  unsigned j;
  for (j = 0; j < N; ++j) {
    counter += j;
  }
}

void foo() {
}

void loop_with_extra_call() {
  unsigned j;
  for (j = 0; j < N; ++j) {
    __asm__("call foo");
    counter += j;
  }
}

int main(int argc, char** argv) {
  if (argc <= 1) {
    return 1;
  }

  if (argv[1][0] == 't') {
    tightloop();
  } else if (argv[1][0] == 'c') {
    loop_with_extra_call();
  }

  return 0;
}

Makefile

CC = gcc

CCOPT = -O2

BUILDDIR = build

all: make_builddir \
	$(BUILDDIR)/loop-call-weirdness

make_builddir:
	@test -d $(BUILDDIR) || mkdir $(BUILDDIR)

$(BUILDDIR)/loop-call-weirdness: loop-call-weirdness.c
	$(CC) $(CCOPT) $^ -o $@

clean:
	rm -rf $(BUILDDIR)/*