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cmd/internal/obj/arm: optimize type promotion #20653

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benshi001 opened this issue Jun 13, 2017 · 5 comments
Closed

cmd/internal/obj/arm: optimize type promotion #20653

benshi001 opened this issue Jun 13, 2017 · 5 comments
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FrozenDueToAge Performance
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Go1.10

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@benshi001
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Currently
var a int32
var b int8
a = int32(b) is built to MOVBS, which actually is
tmp = SLL b, 24
a = SRA tmp, 24

That is quite inefficient.

I suggest use single instruction SXTB which was introduced in ARMv6.
"Signed Extend Byte extracts an 8-bit value from a register, sign-extends it to 32 bits, and writes the result to the destination register."

So do int16->int32(SXTH), uint8->uint32(UXTB), uint16->uint32(UXTH)

It can be done in go1.10 and assigned to me.
@ALTree ALTree added this to the Go1.10 milestone Jun 13, 2017
@bradfitz
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/cc @randall77

@randall77
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Sounds fine to me. The only tricky part would be conditioning it for GOARM>=6.

@benshi001
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benshi001 commented Aug 17, 2017 •
edited

As Cherry @cherrymui suggeusts in https://go-review.googlesource.com/c/53690, more work is needed to introduce XTB/XTBU/XTH/XTHU to golang. Adding a new field in []optab for selecting different instructions for different GOARM version is a good way.

@benshi001
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benshi001 commented Sep 22, 2017 •
edited

@cherrymui
Cherry, I do not think XTB/XTBU/XTH/XTHU can be attributed to MOVB/MOVBU/MOVH/MOVHU,

since there are more complex forms, such as

XTH Rd, Rm, ROR #8

which means signed extension of Rm(23:8) to Rd.

@gopherbot
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Change https://golang.org/cl/71992 mentions this issue: cmd/compile: optimization with more efficient ARMv6 instructions

@golang golang locked and limited conversation to collaborators Oct 26, 2018
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@bradfitz @ALTree @randall77 @gopherbot @benshi001