[Bug tree-optimization/119103] shift not demotated when shift amount range is known

[amonakov at gcc dot gnu.org via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=119103

Alexander Monakov <amonakov at gcc dot gnu.org> changed:

           What    |Removed                     |Added
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                 CC|                            |amonakov at gcc dot gnu.org

--- Comment #15 from Alexander Monakov <amonakov at gcc dot gnu.org> ---
(In reply to Niklas Haas from comment #12)
> Out of curiosity, is there a work-around that I could use to get current
> versions of GCC to compile the right thing, but without breaking
> cross-platform compatibility?
> 
> I did try replacing the assertion by "x[i] << (amount & 0xF)" but the vector
> version of the code at least still compiles down to vpslld.

This quote seems apropos: "autovectorization is not a programming model".

Not sure if you know that already, but with generic vectors you can write:

typedef uint16_t u16v16 __attribute__((vector_size(32)));
typedef u16v16 u16v16_u __attribute__((aligned(2)));

void lshift_register(u16v16_u *x, uint8_t amount)
{
    *x <<= amount;
}