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From: "oneill+gccbugs at cs dot hmc.edu" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug middle-end/62263] Good codegen for bitwise rotate requires code that is technically undefined behavior
Date: Tue, 26 Aug 2014 17:12:00 -0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=62263

--- Comment #6 from M.E. O'Neill <oneill+gccbugs at cs dot hmc.edu> ---
(In reply to Marek Polacek from comment #3)
> We handle at least
>  (x << n) | (x >> ((-n) & 31))
> (N can be 0 here) since PR57157.

Although this code does work with LLVM, testing with GCC 4.9.0, and this
implementation

unsigned int rotl32_doubleand4(unsigned int v, unsigned char r)
{
    return (v << (r & 31)) | (v >> ((-r) & 31));
}

(or variations with r being a char or int) produces code like this:

_rotl32_doubleand4:
LFB6:
    movl    %esi, %ecx
    movl    %edi, %eax
    negl    %ecx
    shrl    %cl, %eax
    movl    %esi, %ecx
    sall    %cl, %edi
    orl    %edi, %eax
    ret

... so it failed to spot the idiom entirely.