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a0453a180f
There are a few cases where this can be useful. Apart from the obvious
(and silly)
100*n + 200*n
where we generate one IMUL instead of two, consider:
15*n + 31*n
Currently, the compiler strength-reduces both imuls, generating:
0x0000 00000 MOVQ "".n+8(SP), AX
0x0005 00005 MOVQ AX, CX
0x0008 00008 SHLQ $4, AX
0x000c 00012 SUBQ CX, AX
0x000f 00015 MOVQ CX, DX
0x0012 00018 SHLQ $5, CX
0x0016 00022 SUBQ DX, CX
0x0019 00025 ADDQ CX, AX
0x001c 00028 MOVQ AX, "".~r1+16(SP)
0x0021 00033 RET
But combining the imuls is both faster and shorter:
0x0000 00000 MOVQ "".n+8(SP), AX
0x0005 00005 IMULQ $46, AX
0x0009 00009 MOVQ AX, "".~r1+16(SP)
0x000e 00014 RET
even without strength-reduction.
Moreover, consider:
5*n + 7*(n+1) + 11*(n+2)
We already have a rule that rewrites 7(n+1) into 7n+7, so the
generated code (without imuls merging) looks like this:
0x0000 00000 MOVQ "".n+8(SP), AX
0x0005 00005 LEAQ (AX)(AX*4), CX
0x0009 00009 MOVQ AX, DX
0x000c 00012 NEGQ AX
0x000f 00015 LEAQ (AX)(DX*8), AX
0x0013 00019 ADDQ CX, AX
0x0016 00022 LEAQ (DX)(CX*2), CX
0x001a 00026 LEAQ 29(AX)(CX*1), AX
0x001f 00031 MOVQ AX, "".~r1+16(SP)
But with imuls merging, the 5n, 7n and 11n factors get merged, and the
generated code looks like this:
0x0000 00000 MOVQ "".n+8(SP), AX
0x0005 00005 IMULQ $23, AX
0x0009 00009 ADDQ $29, AX
0x000d 00013 MOVQ AX, "".~r1+16(SP)
0x0012 00018 RET
Which is both faster and shorter; that's also the exact same code that
clang and the intel c compiler generate for the above expression.
Change-Id: Ib4d5503f05d2f2efe31a1be14e2fe6cac33730a9
Reviewed-on: https://go-review.googlesource.com/55143
Reviewed-by: Keith Randall <khr@golang.org>
82 lines
2.0 KiB
Go
82 lines
2.0 KiB
Go
// runoutput
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// Copyright 2017 The Go Authors. All rights reserved.
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// Use of this source code is governed by a BSD-style
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// license that can be found in the LICENSE file.
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package main
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import "fmt"
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// Check that expressions like (c*n + d*(n+k)) get correctly merged by
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// the compiler into (c+d)*n + d*k (with c+d and d*k computed at
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// compile time).
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//
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// The merging is performed by a combination of the multiplication
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// merge rules
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// (c*n + d*n) -> (c+d)*n
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// and the distributive multiplication rules
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// c * (d+x) -> c*d + c*x
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// Generate a MergeTest that looks like this:
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//
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// a8, b8 = m1*n8 + m2*(n8+k), (m1+m2)*n8 + m2*k
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// if a8 != b8 {
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// // print error msg and panic
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// }
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func makeMergeTest(m1, m2, k int, size string) string {
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model := " a" + size + ", b" + size
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model += fmt.Sprintf(" = %%d*n%s + %%d*(n%s+%%d), (%%d+%%d)*n%s + (%%d*%%d)", size, size, size)
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test := fmt.Sprintf(model, m1, m2, k, m1, m2, m2, k)
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test += fmt.Sprintf(`
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if a%s != b%s {
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fmt.Printf("MergeTest(%d, %d, %d, %s) failed\n")
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fmt.Printf("%%d != %%d\n", a%s, b%s)
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panic("FAIL")
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}
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`, size, size, m1, m2, k, size, size, size)
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return test + "\n"
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}
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func makeAllSizes(m1, m2, k int) string {
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var tests string
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tests += makeMergeTest(m1, m2, k, "8")
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tests += makeMergeTest(m1, m2, k, "16")
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tests += makeMergeTest(m1, m2, k, "32")
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tests += makeMergeTest(m1, m2, k, "64")
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tests += "\n"
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return tests
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}
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func main() {
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fmt.Println(`package main
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import "fmt"
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var n8 int8 = 42
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var n16 int16 = 42
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var n32 int32 = 42
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var n64 int64 = 42
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func main() {
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var a8, b8 int8
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var a16, b16 int16
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var a32, b32 int32
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var a64, b64 int64
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`)
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fmt.Println(makeAllSizes(03, 05, 0)) // 3*n + 5*n
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fmt.Println(makeAllSizes(17, 33, 0))
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fmt.Println(makeAllSizes(80, 45, 0))
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fmt.Println(makeAllSizes(32, 64, 0))
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fmt.Println(makeAllSizes(7, 11, +1)) // 7*n + 11*(n+1)
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fmt.Println(makeAllSizes(9, 13, +2))
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fmt.Println(makeAllSizes(11, 16, -1))
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fmt.Println(makeAllSizes(17, 9, -2))
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fmt.Println("}")
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}
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