unroll: add generic loop unroll helpers

There are cases when we need to explicitly unroll loops. For example,
cache operations, filling DMA descriptors on very high speeds etc.
Add compiler-specific

unroll: add generic loop unroll helpers

There are cases when we need to explicitly unroll loops. For example,
cache operations, filling DMA descriptors on very high speeds etc.
Add compiler-specific attribute macros to give the compiler a hint
that we'd like to unroll a loop.
Example usage:

#define UNROLL_BATCH 8

unrolled_count(UNROLL_BATCH)
for (u32 i = 0; i < UNROLL_BATCH; i++)
op(priv, i);

Note that sometimes the compilers won't unroll loops if they think this
would have worse optimization and perf than without unrolling, and that
unroll attributes are available only starting GCC 8. For older compiler
versions, no hints/attributes will be applied.
For better unrolling/parallelization, don't have any variables that
interfere between iterations except for the iterator itself.

Co-developed-by: Jose E. Marchesi <[email protected]> # pragmas
Signed-off-by: Jose E. Marchesi <[email protected]>
Reviewed-by: Przemek Kitszel <[email protected]>
Signed-off-by: Alexander Lobakin <[email protected]>
Link: https://patch.msgid.link/[email protected]
Signed-off-by: Jakub Kicinski <[email protected]>

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kernel: Add helper macros for loop unrolling

This helps in easily initializing blocks of code (e.g. static calls and
keys).

UNROLL(N, MACRO, __VA_ARGS__) calls MACRO N times with the first
argument

kernel: Add helper macros for loop unrolling

This helps in easily initializing blocks of code (e.g. static calls and
keys).

UNROLL(N, MACRO, __VA_ARGS__) calls MACRO N times with the first
argument as the index of the iteration. This allows string pasting to
create unique tokens for variable names, function calls etc.

As an example:

#include <linux/unroll.h>

#define MACRO(N, a, b) \
int add_##N(int a, int b) \
{ \
return a + b + N; \
}

UNROLL(2, MACRO, x, y)

expands to:

int add_0(int x, int y)
{
return x + y + 0;
}

int add_1(int x, int y)
{
return x + y + 1;
}

Tested-by: Guenter Roeck <[email protected]>
Reviewed-by: Kees Cook <[email protected]>
Reviewed-by: Casey Schaufler <[email protected]>
Reviewed-by: John Johansen <[email protected]>
Acked-by: Jiri Olsa <[email protected]>
Acked-by: Song Liu <[email protected]>
Acked-by: Andrii Nakryiko <[email protected]>
Acked-by: Casey Schaufler <[email protected]>
Nacked-by: Tetsuo Handa <[email protected]>
Signed-off-by: KP Singh <[email protected]>
Signed-off-by: Paul Moore <[email protected]>

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