What is the purpose of bitshifting by 0

Question

I am reviewing the open source AMD GPU drivers for Linux. I noticed something I haven't seen before, and I would like to know the purpose. On line 1441 of the sid.h file, there are a series of defines that are integers being bitshifted left by 0. Wouldn't this just result in the original integer being operated on?

Here is an excerpt and a link to the head

    #define VGT_EVENT_INITIATOR                      0xA2A4
    #define SAMPLE_STREAMOUTSTATS1                   (1 << 0)
    #define SAMPLE_STREAMOUTSTATS2                   (2 << 0)
    #define SAMPLE_STREAMOUTSTATS3                   (3 << 0)

https://github.com/torvalds/linux/blob/master/drivers/gpu/drm/amd/amdgpu/sid.h#L1441

Also, I am learning to access the performance counter registers of AMD GPUs in order to calculate the GPU load. Any tips on that would be appreciated as well.

Answer 1

Things like that could be done just for the sake of consistency (not necessarily applicable to your specific case). For example, I can describe a set of single-bit flags as

#define FLAG_1 0x01
#define FLAG_2 0x02
#define FLAG_3 0x04
#define FLAG_4 0x08

or as

#define FLAG_1 (1u << 0)
#define FLAG_2 (1u << 1)
#define FLAG_3 (1u << 2)
#define FLAG_4 (1u << 3)

In the first line of the latter approach I did not have to shift by 0. But it just looks more consistent that way and emphasizes the fact that FLAG_1 has the same nature as the rest of the flags. And 0 acts as a placeholder for a different value, if I some day decide to change it.

You can actually see exactly that in the linked code with shift by 0 in the definitions of DYN_OR_EN and DYN_RR_EN macros.

---

The approach can be extended to multi-bit fields within a word, like in the following (contrived) example

// Bits 0-3 - lower counter, bits 4-7 - upper counter

#define LOWER_0  (0u << 0)
#define LOWER_1  (1u << 0)
#define LOWER_2  (2u << 0)
#define LOWER_3  (3u << 0)

#define UPPER_0  (0u << 4)
#define UPPER_1  (1u << 4)
#define UPPER_2  (2u << 4)
#define UPPER_3  (3u << 4)

unsigned packed_counters = LOWER_2 + UPPER_3; /* or `LOWER_2 | UPPER_3` */

Again, shifts by 0 bits are present purely for visual consistency. As well as shifts of 0 values.

You can actually see exactly that in the linked code with shift by 0 in the definitions of LC_XMIT_N_FTS and LC_XMIT_N_FTS_MASK macros.