amd64/isel: fix floating point == and != result with NaN

On x86_64, ucomis[sd] sets ZF=1, PF=0, CF=0 for equal arguments.
However, if the arguments are unordered it sets ZF=1, PF=1, CF=1,
and there is no jump/flag instruction for ZF=1 & PF=0 or ZF=1 & CF=0.

So, in order to correctly implement ceq[sd] on x86_64, we need to
be a bit more creative. There are several options available, depending
on whether the result of ceq[sd] is used with jnz, or with other
instructions, or both.

If the result is used for a conditional jump, both gcc and clang
use a combination of jp and jnz:

	ucomisd %xmm1, %xmm0
	jp .Lfalse
	jnz .Lfalse
	...
	.Lfalse:

If the result is used in other instructions or return, gcc does the
following for x == y:

	ucomisd %xmm1, %xmm0
	setnp %al
	movzbl %al, %eax
	movl $0, %edx
	cmovne %edx, %eax

This sets EAX to PF=0, then uses cmovne to clear it if ZF=0. It
also takes care to avoid clobbering the flags register in case the
result is also used for a conditional jump. Implementing this
approach in QBE would require adding an architecture-specific
instruction for cmovne.

In contrast, clang does an additional compare, this time using
cmpeqsd instead of ucomisd:

	cmpeqsd %xmm1, %xmm0
	movq %xmm0, %rax
	andl $1, %rax

The cmpeqsd instruction doas a floating point equality test, setting
XMM0 to all 1s if they are equal and all 0s if they are not. However,
we need the result in a non-XMM register, so it moves the result
back then masks off all but the first bit.

Both of these approaches are a bit awkward to implement in QBE, so
instead, this commit does the following:

	ucomisd %xmm1, %xmm0
	setz %al
	movzbl %al, %eax
	setnp %cl
	movzbl %cl, %ecx
	andl %ecx, %eax

This sets the result by anding the two flags, but has a side effect
of clobbering the flags register. This was a problem in one of my
earlier patches to fix this issue[0], in addition to being more
complex than I'd hoped.

Instead, this commit always leaves the ceq[sd] instruction in the
block, even if the result is only used to control a jump, so that
the above instruction sequence is always used. Then, since we now
have ZF=!(ZF=1 & PF=0) for x == y, or ZF=!(ZF=0 | PF=1) for x != y,
we can use jnz for the jump instruction.

[0] https://git.sr.ht/~sircmpwn/qbe/commit/64833841b18c074a23b4a1254625315e05b86658

0 files changed, 0 insertions, 0 deletions