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docs/LangRef.rst
2347	This is precisely the definition of undef: an arbitrary value that may yield a different value each time it's used. But the intention was to avoid undef and poison. I'm not familiar enough with floats to make any concrete suggestion; I'm just type checking.

Clarify that the value produced isn't undef. (It's more like freeze(undef).)

This reads clearer to me by avoiding the potential to confuse with UB. Should the same blurb apply to nsz with -0.0?

I don't think this is right. Saying the result is undefined seems like what we intend. We might choose, as an implementation technique, to limit how we take advantage of that undefinedness in certain cases, but the violating the constraint still produces logical inconsistencies that can transfer to other parts of the code, and in general, turn into any other kind of undefined behavior (depending on the structure of the code).

It has to be one of undefined behavior, poison, undef, or an unspecified value as described here; there is no other way for something to be "undefined" in LLVM IR, unless you're proposing to introduce a new form of undefined-ness specifically for nnan/ninf.

I think we should have these produce poison -- if we have them produce undef we'll have the same kinds of problems as we have with regular undef.

nnan and ninf affect fcmp

Only if the fcmp is marked with nnan or ninf right?

a.elovikov added a subscriber: a.elovikov.Jun 18 2018, 12:19 AM

Only if the fcmp is marked with nnan or ninf right?

Yes, but it's inconvenient in C because fast-math flags work on a per-file basis.

If the consensus is that we need to produce poison for nnan/ninf instead of an unspecified value, I can change that. I guess it allows more aggressive fcmp transforms in some cases?

Okay, switched to poison.

spatel mentioned this in rL336562: [InstCombine] avoid extra poison when moving shift above shuffle.Jul 9 2018, 10:25 AM

Ping

LGTM

This revision is now accepted and ready to land.Jul 13 2018, 7:57 PM

Closed by commit rL337323: [LangRef] nnan and ninf produce poison. (authored by efriedma). · Explain WhyJul 17 2018, 1:36 PM

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	:ref:`fsub <i_fsub>`, :ref:`fmul <i_fmul>`, :ref:`fdiv <i_fdiv>`,			:ref:`fsub <i_fsub>`, :ref:`fmul <i_fmul>`, :ref:`fdiv <i_fdiv>`,
	:ref:`frem <i_frem>`, :ref:`fcmp <i_fcmp>`) and :ref:`call <i_call>`			:ref:`frem <i_frem>`, :ref:`fcmp <i_fcmp>`) and :ref:`call <i_call>`
	may use the following flags to enable otherwise unsafe			may use the following flags to enable otherwise unsafe
	floating-point transformations.			floating-point transformations.

	``nnan``			``nnan``
	No NaNs - Allow optimizations to assume the arguments and result are not			No NaNs - Allow optimizations to assume the arguments and result are not
	NaN. Such optimizations are required to retain defined behavior over			NaN. Such optimizations are required to retain defined behavior over
	NaNs, but the value of the result is undefined.			NaNs, but the value of the result is unspecified. The unspecified result
				may not be the same each time the instruction is executed.
				nlopesUnsubmitted Not Done Reply Inline Actions This is precisely the definition of undef: an arbitrary value that may yield a different value each time it's used. But the intention was to avoid undef and poison. I'm not familiar enough with floats to make any concrete suggestion; I'm just type checking. nlopes: This is precisely the definition of undef: an arbitrary value that may yield a different value…

	``ninf``			``ninf``
	No Infs - Allow optimizations to assume the arguments and result are not			No Infs - Allow optimizations to assume the arguments and result are not
	+/-Inf. Such optimizations are required to retain defined behavior over			+/-Inf. Such optimizations are required to retain defined behavior over
	+/-Inf, but the value of the result is undefined.			+/-Inf, but the value of the result is unspecified. The unspecified result
				may not be the same each time the instruction is executed.

	``nsz``			``nsz``
	No Signed Zeros - Allow optimizations to treat the sign of a zero			No Signed Zeros - Allow optimizations to treat the sign of a zero
	argument or result as insignificant.			argument or result as insignificant.

	``arcp``			``arcp``
	Allow Reciprocal - Allow optimizations to use the reciprocal of an			Allow Reciprocal - Allow optimizations to use the reciprocal of an
	argument rather than perform division.			argument rather than perform division.
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