Lioncash 0d8ef2d3b9 common/swap: Improve codegen of the default swap fallbacks Uses arithmetic that can be identified more trivially by compilers for optimizations. e.g. Rather than shifting the halves of the value and then swapping and combining them, we can swap them in place. e.g. for the original swap32 code on x86-64, clang 8.0 would generate: mov ecx, edi rol cx, 8 shl ecx, 16 shr edi, 16 rol di, 8 movzx eax, di or eax, ecx ret while GCC 8.3 would generate the ideal: mov eax, edi bswap eax ret now both generate the same optimal output. MSVC used to generate the following with the old code: mov eax, ecx rol cx, 8 shr eax, 16 rol ax, 8 movzx ecx, cx movzx eax, ax shl ecx, 16 or eax, ecx ret 0 Now MSVC also generates a similar, but equally optimal result as clang/GCC: bswap ecx mov eax, ecx ret 0 ==== In the swap64 case, for the original code, clang 8.0 would generate: mov eax, edi bswap eax shl rax, 32 shr rdi, 32 bswap edi or rax, rdi ret (almost there, but still missing the mark) while, again, GCC 8.3 would generate the more ideal: mov rax, rdi bswap rax ret now clang also generates the optimal sequence for this fallback as well. This is a case where MSVC unfortunately falls short, despite the new code, this one still generates a doozy of an output. mov r8, rcx mov r9, rcx mov rax, 71776119061217280 mov rdx, r8 and r9, rax and edx, 65280 mov rax, rcx shr rax, 16 or r9, rax mov rax, rcx shr r9, 16 mov rcx, 280375465082880 and rax, rcx mov rcx, 1095216660480 or r9, rax mov rax, r8 and rax, rcx shr r9, 16 or r9, rax mov rcx, r8 mov rax, r8 shr r9, 8 shl rax, 16 and ecx, 16711680 or rdx, rax mov eax, -16777216 and rax, r8 shl rdx, 16 or rdx, rcx shl rdx, 16 or rax, rdx shl rax, 8 or rax, r9 ret 0 which is pretty unfortunate.	8 years ago
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README.md	Add submodules	8 years ago
json.hpp	Add submodules	8 years ago

Lioncash 0d8ef2d3b9 common/swap: Improve codegen of the default swap fallbacks

Uses arithmetic that can be identified more trivially by compilers for
optimizations. e.g. Rather than shifting the halves of the value and
then swapping and combining them, we can swap them in place.

e.g. for the original swap32 code on x86-64, clang 8.0 would generate:

    mov     ecx, edi
    rol     cx, 8
    shl     ecx, 16
    shr     edi, 16
    rol     di, 8
    movzx   eax, di
    or      eax, ecx
    ret

while GCC 8.3 would generate the ideal:

    mov     eax, edi
    bswap   eax
    ret

now both generate the same optimal output.

MSVC used to generate the following with the old code:

    mov     eax, ecx
    rol     cx, 8
    shr     eax, 16
    rol     ax, 8
    movzx   ecx, cx
    movzx   eax, ax
    shl     ecx, 16
    or      eax, ecx
    ret     0

Now MSVC also generates a similar, but equally optimal result as clang/GCC:

    bswap   ecx
    mov     eax, ecx
    ret     0

====

In the swap64 case, for the original code, clang 8.0 would generate:

    mov     eax, edi
    bswap   eax
    shl     rax, 32
    shr     rdi, 32
    bswap   edi
    or      rax, rdi
    ret

(almost there, but still missing the mark)

while, again, GCC 8.3 would generate the more ideal:

    mov     rax, rdi
    bswap   rax
    ret

now clang also generates the optimal sequence for this fallback as well.

This is a case where MSVC unfortunately falls short, despite the new
code, this one still generates a doozy of an output.

    mov     r8, rcx
    mov     r9, rcx
    mov     rax, 71776119061217280
    mov     rdx, r8
    and     r9, rax
    and     edx, 65280
    mov     rax, rcx
    shr     rax, 16
    or      r9, rax
    mov     rax, rcx
    shr     r9, 16
    mov     rcx, 280375465082880
    and     rax, rcx
    mov     rcx, 1095216660480
    or      r9, rax
    mov     rax, r8
    and     rax, rcx
    shr     r9, 16
    or      r9, rax
    mov     rcx, r8
    mov     rax, r8
    shr     r9, 8
    shl     rax, 16
    and     ecx, 16711680
    or      rdx, rax
    mov     eax, -16777216
    and     rax, r8
    shl     rdx, 16
    or      rdx, rcx
    shl     rdx, 16
    or      rax, rdx
    shl     rax, 8
    or      rax, r9
    ret     0

which is pretty unfortunate.

8 years ago

README.md

Add submodules

8 years ago

json.hpp

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8 years ago

README.md

JSON for Modern C++

v3.1.2

This is a mirror providing the single required header file.

The original repository can be found at: d2dd27dc3b