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cmd/compile: optimize away useless ANDs and ORs #14367

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dsnet opened this issue Feb 17, 2016 · 5 comments
Closed

cmd/compile: optimize away useless ANDs and ORs #14367

dsnet opened this issue Feb 17, 2016 · 5 comments
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FrozenDueToAge Performance
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Go1.7

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@dsnet
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dsnet commented Feb 17, 2016

Using go1.5.3

The following code has a few useless expressions:

return 0 | // Useless OR with 0
    ((n & 0xff) << 56) | // Useless AND with 0xff followed by left-shift 56
    ((n & 0xff00) << 40) |
    ((n & 0xff0000) << 24) |
    ((n & 0xff000000) << 8) |
    ((n & 0xff00000000) >> 8) |
    ((n & 0xff0000000000) >> 24) |
    ((n & 0xff000000000000) >> 40) |
    ((n & 0xff00000000000000) >> 56) // Useless AND with 0xff00... followed by right-shift 56

The instructions with # marks are unnecessary. It would be nice if the compiler did not generate them.

"".bswap64a t=1 size=160 value=0 args=0x10 locals=0x0
    ...
    0x0005 00005 (byteswap.go:14)    MOVQ    AX, BX
#   0x0008 00008 (byteswap.go:14)    ANDQ    $255, BX
    0x000f 00015 (byteswap.go:14)    SHLQ    $56, BX
#   0x0013 00019 (byteswap.go:14)    ORQ $0, BX
    0x0017 00023 (byteswap.go:14)    MOVQ    AX, BP
    0x001a 00026 (byteswap.go:14)    ANDQ    $65280, BP
    0x0021 00033 (byteswap.go:14)    SHLQ    $40, BP
    0x0025 00037 (byteswap.go:14)    ORQ BP, BX
    0x0028 00040 (byteswap.go:14)    MOVQ    AX, BP
    0x002b 00043 (byteswap.go:14)    ANDQ    $16711680, BP
    0x0032 00050 (byteswap.go:14)    SHLQ    $24, BP
    0x0036 00054 (byteswap.go:14)    ORQ BP, BX
    0x0039 00057 (byteswap.go:14)    MOVQ    $4278190080, BP
    0x003e 00062 (byteswap.go:14)    ANDQ    AX, BP
    0x0041 00065 (byteswap.go:14)    SHLQ    $8, BP
    0x0045 00069 (byteswap.go:14)    ORQ BP, BX
    0x0048 00072 (byteswap.go:14)    MOVQ    $1095216660480, BP
    0x0052 00082 (byteswap.go:14)    ANDQ    AX, BP
    0x0055 00085 (byteswap.go:14)    SHRQ    $8, BP
    0x0059 00089 (byteswap.go:14)    ORQ BP, BX
    0x005c 00092 (byteswap.go:14)    MOVQ    $280375465082880, BP
    0x0066 00102 (byteswap.go:14)    ANDQ    AX, BP
    0x0069 00105 (byteswap.go:14)    SHRQ    $24, BP
    0x006d 00109 (byteswap.go:14)    ORQ BP, BX
    0x0070 00112 (byteswap.go:14)    MOVQ    $71776119061217280, BP
    0x007a 00122 (byteswap.go:14)    ANDQ    AX, BP
    0x007d 00125 (byteswap.go:14)    SHRQ    $40, BP
    0x0081 00129 (byteswap.go:14)    ORQ BP, BX
#   0x0084 00132 (byteswap.go:14)    MOVQ    $-72057594037927936, BP
    0x008e 00142 (byteswap.go:14)    ANDQ    AX, BP
    0x0091 00145 (byteswap.go:14)    SHRQ    $56, BP
    0x0095 00149 (byteswap.go:14)    ORQ BP, BX
    0x0098 00152 (byteswap.go:14)    MOVQ    BX, "".~r1+16(FP)
    0x009d 00157 (byteswap.go:14)    RET
@bradfitz
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Have you tried the ssa branch?

@dsnet
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dsnet commented Feb 17, 2016

Just tried it on dev.ssa bc1fb32

It elides instructions for OR zero, but strangely does a triple shift in some places:

"".byteswap t=1 size=160 value=0 args=0x10 locals=0x0
    ...
    0x0005 00005 (byteswap.go:5)    MOVQ    AX, CX
    0x0008 00008 (byteswap.go:5)    SHLQ    $56, CX
#   0x000c 00012 (byteswap.go:5)    SHRQ    $56, CX
#   0x0010 00016 (byteswap.go:5)    SHLQ    $56, CX
    0x0014 00020 (byteswap.go:6)    MOVQ    AX, DX
    0x0017 00023 (byteswap.go:6)    ANDQ    $65280, DX
    0x001e 00030 (byteswap.go:6)    SHLQ    $40, DX
    0x0022 00034 (byteswap.go:6)    ORQ DX, CX
    0x0025 00037 (byteswap.go:7)    MOVQ    AX, DX
    0x0028 00040 (byteswap.go:7)    ANDQ    $16711680, DX
    0x002f 00047 (byteswap.go:7)    SHLQ    $24, DX
    0x0033 00051 (byteswap.go:7)    ORQ DX, CX
    0x0036 00054 (byteswap.go:8)    MOVQ    $4278190080, DX
    0x003b 00059 (byteswap.go:8)    ANDQ    AX, DX
    0x003e 00062 (byteswap.go:8)    SHLQ    $8, DX
    0x0042 00066 (byteswap.go:8)    ORQ DX, CX
    0x0045 00069 (byteswap.go:9)    MOVQ    $1095216660480, DX
    0x004f 00079 (byteswap.go:9)    ANDQ    AX, DX
    0x0052 00082 (byteswap.go:9)    SHRQ    $8, DX
    0x0056 00086 (byteswap.go:9)    ORQ DX, CX
    0x0059 00089 (byteswap.go:10)   MOVQ    $280375465082880, DX
    0x0063 00099 (byteswap.go:10)   ANDQ    AX, DX
    0x0066 00102 (byteswap.go:10)   SHRQ    $24, DX
    0x006a 00106 (byteswap.go:10)   ORQ DX, CX
    0x006d 00109 (byteswap.go:11)   MOVQ    $71776119061217280, DX
    0x0077 00119 (byteswap.go:11)   ANDQ    AX, DX
    0x007a 00122 (byteswap.go:11)   SHRQ    $40, DX
    0x007e 00126 (byteswap.go:11)   ORQ DX, CX
    0x0081 00129 (byteswap.go:12)   SHRQ    $56, AX
#   0x0085 00133 (byteswap.go:12)   SHLQ    $56, AX
#   0x0089 00137 (byteswap.go:12)   SHRQ    $56, AX
    0x008d 00141 (byteswap.go:12)   ORQ CX, AX
    0x0090 00144 (byteswap.go:12)   MOVQ    AX, "".~r1+16(FP)
    0x0095 00149 (byteswap.go:12)   RET

@tzneal
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tzneal commented Feb 18, 2016

I'll look at this one, I introduced the AND to double shift :-)

https://go-review.googlesource.com/19485/

@gopherbot
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CL https://golang.org/cl/19690 mentions this issue.

@dsnet dsnet changed the title cmd/compile: optimize away useless expressions cmd/compile: optimize away useless A Feb 19, 2016
@dsnet dsnet changed the title cmd/compile: optimize away useless A cmd/compile: optimize away useless AND and ORs Feb 19, 2016
@dsnet dsnet changed the title cmd/compile: optimize away useless AND and ORs cmd/compile: optimize away useless ANDs and ORs Feb 19, 2016
@bradfitz bradfitz added this to the Go1.7 milestone Apr 10, 2016
@randall77
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The final AND 0xff00000000000000 is gone.
There is a MOVBQZX at the start that seems unnecessary, but it would need to be replaced with a MOVQ anyway, so it doesn't look like a big loss. I'm going to mark this as closed.

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