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cmd/internal/obj/x86: port the doasm comment to go

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This comment is very useful but still refers to the C implementation.
Adapting it for Go is fairly straightforward though.

Change-Id: Ib6dde25f3a18acbce76bb3cffdc29f5ccf43c1f7
Reviewed-on: https://go-review.googlesource.com/41696
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
This commit is contained in:
Damien Lespiau 2017-04-25 16:43:57 +01:00 committed by Brad Fitzpatrick
parent db6f3bbc9a
commit 4e5593ddaa
1 changed files with 19 additions and 21 deletions
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40
src/cmd/internal/obj/x86/asm6.go
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@ -882,19 +882,19 @@ var ymmxmm0f38 = []ytab{
}
/*
* You are doasm, holding in your hand a Prog* with p->as set to, say, ACRC32,
* and p->from and p->to as operands (Addr*). The linker scans optab to find
* the entry with the given p->as and then looks through the ytable for that
* instruction (the second field in the optab struct) for a line whose first
* two values match the Ytypes of the p->from and p->to operands. The function
* oclass in span.c computes the specific Ytype of an operand and then the set
* You are doasm, holding in your hand a *obj.Prog with p.As set to, say,
* ACRC32, and p.From and p.To as operands (obj.Addr). The linker scans optab
* to find the entry with the given p.As and then looks through the ytable for
* that instruction (the second field in the optab struct) for a line whose
* first two values match the Ytypes of the p.From and p.To operands. The
* function oclass computes the specific Ytype of an operand and then the set
* of more general Ytypes that it satisfies is implied by the ycover table, set
* up in instinit. For example, oclass distinguishes the constants 0 and 1
* from the more general 8-bit constants, but instinit says
*
* ycover[Yi0*Ymax + Ys32] = 1;
* ycover[Yi1*Ymax + Ys32] = 1;
* ycover[Yi8*Ymax + Ys32] = 1;
* ycover[Yi0*Ymax+Ys32] = 1
* ycover[Yi1*Ymax+Ys32] = 1
* ycover[Yi8*Ymax+Ys32] = 1
*
* which means that Yi0, Yi1, and Yi8 all count as Ys32 (signed 32)
* if that's what an instruction can handle.
@ -908,22 +908,20 @@ var ymmxmm0f38 = []ytab{
* is, the Ztype) and the z bytes.
*
* For example, let's look at AADDL. The optab line says:
* { AADDL, yaddl, Px, 0x83,(00),0x05,0x81,(00),0x01,0x03 },
* {AADDL, yaddl, Px, [23]uint8{0x83, 00, 0x05, 0x81, 00, 0x01, 0x03}},
*
* and yaddl says
* uchar yaddl[] =
* {
* Yi8, Yml, Zibo_m, 2,
* Yi32, Yax, Zil_, 1,
* Yi32, Yml, Zilo_m, 2,
* Yrl, Yml, Zr_m, 1,
* Yml, Yrl, Zm_r, 1,
* 0
* };
* var yaddl = []ytab{
* {Yi8, Ynone, Yml, Zibo_m, 2},
* {Yi32, Ynone, Yax, Zil_, 1},
* {Yi32, Ynone, Yml, Zilo_m, 2},
* {Yrl, Ynone, Yml, Zr_m, 1},
* {Yml, Ynone, Yrl, Zm_r, 1},
* }
*
* so there are 5 possible types of ADDL instruction that can be laid down, and
* possible states used to lay them down (Ztype and z pointer, assuming z
* points at {0x83,(00),0x05,0x81,(00),0x01,0x03}) are:
* points at [23]uint8{0x83, 00, 0x05,0x81, 00, 0x01, 0x03}) are:
*
* Yi8, Yml -> Zibo_m, z (0x83, 00)
* Yi32, Yax -> Zil_, z+2 (0x05)
@ -934,7 +932,7 @@ var ymmxmm0f38 = []ytab{
* The Pconstant in the optab line controls the prefix bytes to emit. That's
* relatively straightforward as this program goes.
*
* The switch on t[2] in doasm implements the various Z cases. Zibo_m, for
* The switch on yt.zcase in doasm implements the various Z cases. Zibo_m, for
* example, is an opcode byte (z[0]) then an asmando (which is some kind of
* encoded addressing mode for the Yml arg), and then a single immediate byte.
* Zilo_m is the same but a long (32-bit) immediate.