Re: [RFC][PATCH] new byteorder primitives - ..._{replace,get}_bits()

[Jakub Kicinski]

On Wed, 13 Dec 2017 01:51:25 +0000, Al Viro wrote:
> On Tue, Dec 12, 2017 at 05:35:28PM -0800, Jakub Kicinski wrote:
> 
> > It used to be __always_inline, but apparently LLVM/clang doesn't
> > propagate constants :(  
> > 
> > 4e59532541c8 ("nfp: don't depend on compiler constant propagation")  
> 
> Doesn't propagate constants or doesn't have exact same set of
> rules for __builtin_constant_p()?  IOW, if you dropped that
> BUILD_BUG_ON(), what would be left after optimizations?

Hm.  You're right.  It just doesn't recognize the parameter as constant
in __builtin_constant_p().  I haven't compiled the actual code, but
here is a trivial test:

18:36 ~$ clang --version
clang version 4.0.1 (tags/RELEASE_401/final)
Target: x86_64-unknown-linux-gnu
Thread model: posix
InstalledDir: /usr/bin

18:36 ~$ cat /tmp/test.c
#include <stdio.h>

static inline int test_thing(unsigned int a)
{
	printf("const: %d\n", __builtin_constant_p(a));
	if (a > 10)
		return a - 1;
	return a;
}

int main()
{
	printf("const: %d\n", __builtin_constant_p(0));
	return test_thing(0);
}
18:36 ~$ clang -g /tmp/test.c -O2 -Wall -W -o /tmp/test
18:36 ~$ /tmp/test 
const: 1
const: 0
18:36 ~$ objdump -S /tmp/test
...
00000000004004e0 <main>:
	return a;
}

int main()
{
	printf("const: %d\n", __builtin_constant_p(0));
  4004e0:	50                   	push   %rax
  4004e1:	bf a0 05 40 00       	mov    $0x4005a0,%edi
  4004e6:	be 01 00 00 00       	mov    $0x1,%esi
  4004eb:	31 c0                	xor    %eax,%eax
  4004ed:	e8 fe fe ff ff       	callq  4003f0 <printf@plt>
	printf("const: %d\n", __builtin_constant_p(a));
  4004f2:	bf a0 05 40 00       	mov    $0x4005a0,%edi
  4004f7:	31 f6                	xor    %esi,%esi
  4004f9:	31 c0                	xor    %eax,%eax
  4004fb:	e8 f0 fe ff ff       	callq  4003f0 <printf@plt>
	return test_thing(0);
  400500:	31 c0                	xor    %eax,%eax
  400502:	59                   	pop    %rcx
  400503:	c3                   	retq   
  400504:	66 2e 0f 1f 84 00 00 	nopw   %cs:0x0(%rax,%rax,1)
  40050b:	00 00 00 
  40050e:	66 90                	xchg   %ax,%ax

...