On Tue, Jul 6, 2021 at 9:01 PM Roger Sayle <ro...@nextmovesoftware.com> wrote:
>
>
> All of the optimizations/transformations mentioned in bugzilla for
> PR tree-optimization/40210 are already implemented in mainline GCC,
> with one exception. In comment #5, there's a suggestion that
> (bswap64(x)>>56)&0xff can be implemented without the bswap as
> (unsigned char)x, or equivalently x&0xff.
>
> This patch implements the above optimization, and closely related
> variants. For any single bit, (bswap(X)>>C1)&1 can be simplified
> to (X>>C2)&1, where bit position C2 is the appropriate permutation
> of C1. Similarly, the bswap can eliminated if the desired set of
> bits all lie within the same byte, hence (bswap(x)>>8)&255 can
> always be optimized, as can (bswap(x)>>8)&123.
>
> Previously,
>
> int foo(long long x) {
> return (__builtin_bswap64(x) >> 56) & 0xff;
> }
>
> compiled with -O2 to
> foo: movq %rdi, %rax
> bswap %rax
> shrq $56, %rax
> ret
>
> with this patch, it now compiles to
> foo: movzbl %dil, %eax
> ret
>
> This patch has been tested on x86_64-pc-linux-gnu with a "make
> bootstrap" and "make -k check" with no new failures.
>
> Ok for mainline?
I don't like get_builtin_precision too much, did you consider
simply using
+ (bit_and (convert1? (rshift@0 (convert2? (bswap@3 @1)) INTEGER_CST@2))
and TYPE_PRECISION (TREE_TYPE (@3))? I think while we'll
see argument promotion and thus cannot use @1 to derive the type
the return value will be the original type.
Now, I see '8' being used which likely should be CHAR_TYPE_SIZE
since you also use char_type_node.
I wonder whether
+ /* (bswap(x) >> C1) & C2 can sometimes be simplified to (x >> C3) & C2. */
+ (simplify
+ (bit_and (convert1? (rshift@0 (convert2? (bswap @1)) INTEGER_CST@2))
+ INTEGER_CST@3)
and
+ /* bswap(x) >> C1 can sometimes be simplified to (T)x >> C2. */
+ (simplify
+ (rshift (convert? (bswap @0)) INTEGER_CST@1)
can build upon each other, for example by extending the latter
to handle more cases, transforming to ((T)x >> C2) & C3?
That might of course be only profitable when the bswap goes away.
Thanks,
Richard.
>
>
> 2021-07-06 Roger Sayle <ro...@nextmovesoftware.com>
>
> gcc/ChangeLog
> PR tree-optimization/40210
> * builtins.c (get_builtin_precision): Helper function to determine
> the precision in bits of a built-in function.
> * builtins.h (get_builtin_precision): Prototype here.
> * match.pd (bswap optimizations): Simplify (bswap(x)>>C1)&C2 as
> (x>>C3)&C2 when possible. Simplify bswap(x)>>C1 as ((T)x)>>C2
> when possible. Simplify bswap(x)&C1 as (x>>C2)&C1 when 0<=C1<=255.
>
> gcc/testsuite/ChangeLog
> PR tree-optimization/40210
> * gcc.dg/builtin-bswap-13.c: New test.
> * gcc.dg/builtin-bswap-14.c: New test.
>
> Roger
> --
> Roger Sayle
> NextMove Software
> Cambridge, UK
>
