[RFC] Design for flag bit outputs from asms

H. Peter Anvin hpa@zytor.com
Mon May 4 21:23:00 GMT 2015

On 05/04/2015 01:57 PM, Richard Henderson wrote:
> Sure.
> I'd be more inclined to support these compound conditionals directly, rather
> than try to get the compiler to recognize them after the fact.
> Indeed, I believe we have a near complete set of them in the x86 backend
> already.  It'd just be a matter of selecting the spellings for the constraints.

Whichever works for you.

The full set of conditions, mnemonics, and a bitmask with the bits in
the order from MSB to LSB (OF,SF,ZF,PF,CF) which is probably the sanest
way to model these for the purpose of boolean optimization.

Opcode	Mnemonics	Condition		Bitmask
0	o		OF			0xffff0000
1	no		!OF			0x0000ffff
2	b/c/nae		CF			0xaaaaaaaa
3	ae/nb/nc	!CF			0x55555555
4	e/z		ZF			0xf0f0f0f0
5	ne/nz		!ZF			0x0f0f0f0f
6	na		CF || ZF		0xfafafafa
7	a		!CF && !ZF		0x05050505
8	s		SF			0xff00ff00
9	ns		!SF			0x00ff00ff
A	p/pe		PF			0xcccccccc
B	np/po		!PF			0x33333333
C	l/nge		SF != OF		0x00ffff00
D	ge/nl		SF == OF		0xff0000ff
E	le/ng		ZF || (SF != OF)	0xf0fffff0
F	g/nle		!ZF && (SF == OF)	0x0f00000f


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