[llvm-dev] RFC: Adding 'no-overflow' keyword to 'sdiv'\'udiv'instructions

Wed Dec 6 23:33:08 PST 2017

>Your proposal is essentially to introduce division instructions that cannot trigger UB, but return poison instead. On ISAs like x86 that means that these instructions have to be >lowered with guards around them.

We return a poison value when this flag is not present. And yes in X86 we can simulate this by having a guard around div instruction (to make sure no trap can be raised) but there is another way to implement that using fp div in x86 once it becomes profitable.

>You also propose to change clang to always emit these non-UB-triggering instructions.  

Clang behavior should be the same like the current behavior which means undefined behavior in case of divide-by-zero/overflow (currently this raise a trap on X86) so to keep this behavior clang should generate div instruction with the flag set by default (i.e. div nof ...).

> Is this only for vector operations or also for scalar ones? 
>What's the performance impact of all those extra guards?

The keyword is for both scalar and vector div instructions.
Of course these guards may hit the performance but this will allow the compiler to hoist\speculate div instructions which can be profitable in case of vectorization in some cases for example.

> Also, if your ISA has vector instructions that don't trigger UB on e.g. 
>division by zero, why don't you rely on this target-specific information in the vectorizer instead?  I mean, you would still need to add the attribute you are proposing, but you wouldn't change clang.

I am not sure that I got your point here, but we need this in order to know that the code may have div-by-zero which didn't come from the original code (speculation) so once the vectorizer tries to 
Widen predicated div instruction it should delete the 'nof' flag if exist. Each target should choose how to lower this instruction and generating this widened div still will be controlled by the vectorizer cost-model.

-----Original Message-----
From: Nuno Lopes [mailto:nunoplopes at sapo.pt] 
Sent: Sunday, December 03, 2017 16:57
To: Agabaria, Mohammed <mohammed.agabaria at intel.com>; llvm-dev at lists.llvm.org
Subject: Re: [llvm-dev] RFC: Adding 'no-overflow' keyword to 'sdiv'\'udiv'instructions

Your proposal is essentially to introduce division instructions that cannot trigger UB, but return poison instead. On ISAs like x86 that means that these instructions have to be lowered with guards around them.
You also propose to change clang to always emit these non-UB-triggering instructions.  Is this only for vector operations or also for scalar ones? 
What's the performance impact of all those extra guards?

Also, if your ISA has vector instructions that don't trigger UB on e.g. 
division by zero, why don't you rely on this target-specific information in the vectorizer instead?  I mean, you would still need to add the attribute you are proposing, but you wouldn't change clang.

Nuno

-----Original Message-----
From: Agabaria, Mohammed via llvm-dev
Sent: Wednesday, November 29, 2017 9:23 AM
To: llvm-dev at lists.llvm.org
Subject: [llvm-dev] RFC: Adding 'no-overflow' keyword to 'sdiv'\'udiv'instructions

Introduction:

We would like to add new keyword to 'sdiv'\'udiv' instructions i.e. 
'no-overflow'.

This is the updated solution devised in the discussion: 
http://lists.llvm.org/pipermail/llvm-dev/2017-October/118257.html

The proposed keywords:

    "nof" stands for 'no-overflow'

Syntax:

    <result> = sdiv nof <ty> <op1>, <op2> ; yields ty:result

    <result> = udiv nof <ty> <op1>, <op2> ; yields ty:result

Overview:

If the keyword is present, the compiler can assume no zero values in the denominator. Moreover, for sdiv the division MIN_INT / -1 is prohibited. 
Otherwise, undefined behavior.

Poison value is returned, in case of division by zero or MIN_INT/-1 if the 
keyword not present.

Motivation:

In the current state if the loop-vectorizer decides that it should vectorize 
a loop which contains a predicated integer division -  it will vectorize the 
loop body and scalarize the predicated division instruction into a sequence 
of branches that guard scalar division operations. In some cases the 
generated code for this will not be very efficient. Speculating the divides 
using current vector sdiv instruction is not an option due to the danger of 
integer divide-by-zero.

There are two ways for ensuring the safety of "vector div under condition", 
One way is to use the same condition as the scalar execution. Current 
serialization approach and previous masked integer div intrinsic proposal 
(http://lists.llvm.org/pipermail/llvm-dev/2017-October/118257.html) follows 
this idea. Second way is to check the actual divisor, regardless of the 
original condition. The 'no-overflow' keyword follows this idea. If the 
original code has possible div-by-zero behavior, for example, the latter 
approach will end up hiding it -- by taking advantage of the undefined 
behavior.

With the addition of 'nof' keyword Clang will lower C\C++ division to 'nof' 
div IR since it will keep the same semantics.

In case the vectorizer decided to vectorize one of the predicated div it can 
be done by widening the datatype of the div and the 'nof' keyword will not 
hold anymore (because of the risk that one of the predicated lanes may have 
zero).

Keeping that with the widened datatype will allow codegen to lower that 
instruction as a vector instruction while ensuring lanes that may have zero 
values do not trigger a trap.

Implementation considerations:

Initially all the targets can scalarize vector sdiv\udiv instructions to one 
with 'nof' by using guards for each lane:

%r = sdiv <4 x i32> %a, %b can be lowered to:

(assuimg %a = <i32 %a.0, i32 %a.1, i32 %a.2, i32 %a.3>, %b = <i32 %b.0, i32 
%b.1, i32 %b.2, i32 %b.3> and %r = <i32 %r.0, i32 %r.1, i32 %r.2, i32 %r.3>)

If CheckSafety(%a.0,%b.0):

  %r.0 = sdiv nof i32 %a.0, %b.0

If CheckSafety(%a.1,%b.1):

  %r.1 = sdiv nof i32 %a.1, %b.1

If CheckSafety(%a.2,%b.2):

  %r.2 = sdiv nof i32 %a.2, %b.2

If CheckSafety(%a.3,%b.3):

  %r.3 = sdiv nof i32 %a.3, %b.3

CheckSafety(a,b): (of sdiv)

  b != 0 || (b != -1 && a != MIN_INT)

CheckSafety(a,b): (of udiv)

  b != 0

Changes in LangRef.rst of udiv/sdiv Instructions:

-----------------------------------------------------------

'``udiv``' Instruction

^^^^^^^^^^^^^^^

Syntax:

"""""""

::

       <result> = udiv <ty> <op1>, <op2>         ; yields ty:result

       <result> = udiv exact <ty> <op1>, <op2>   ; yields ty:result

+       <result> = udiv nof <ty> <op1>, <op2>   ; yields ty:result

Overview:

"""""""""

The '``udiv``' instruction returns the quotient of its two operands.

Arguments:

""""""""""

The two arguments to the '``udiv``' instruction must be

:ref:`integer <t_integer>` or :ref:`vector <t_vector>` of integer values. 
Both

arguments must have identical types.

Semantics:

""""""""""

The value produced is the unsigned integer quotient of the two operands.

Note that unsigned integer division and signed integer division are

distinct operations; for signed integer division, use '``sdiv``'.

Division by zero is undefined behavior. For vectors, if any element

of the divisor is zero, the operation has undefined behavior.

See the description of the ``nof`` keyword below for division by zero.

If the ``exact`` keyword is present, the result value of the ``udiv`` is

a :ref:`poison value <poisonvalues>` if %op1 is not a multiple of %op2 (as

such, "((a udiv exact b) mul b) == a").

``nof``stands for “No Overflow”. If the ``nof`` keyword is present, the 
result is undefined behavior for division by zero.

If the ``nof`` keyword is not present, division by zero results in poison 
value.

For vectors, if any element of the divisor is zero, the behavior is same as 
for scalar division by zero.

'``sdiv``' Instruction

^^^^^^^^^^^^^^^

Syntax:

"""""""

::

       <result> = sdiv <ty> <op1>, <op2>         ; yields ty:result

       <result> = sdiv exact <ty> <op1>, <op2>   ; yields ty:result

+       <result> = sdiv nof <ty> <op1>, <op2>   ; yields ty:result

Overview

"""""""""

The '``sdiv``' instruction returns the quotient of its two operands.

Arguments:

""""""""""

The two arguments to the '``sdiv``' instruction must be

:ref:`integer <t_integer>` or :ref:`vector <t_vector>` of integer values. 
Both

arguments must have identical types.

Semantics:

""""""""""

The value produced is the signed integer quotient of the two operands

rounded towards zero.

Note that signed integer division and unsigned integer division are

distinct operations; for unsigned integer division, use '``udiv``'.

Division by zero is undefined behavior. For vectors, if any element

of the divisor is zero, the operation has undefined behavior.

Overflow also leads to undefined behavior; this is a rare case, but can

occur, for example, by doing a 32-bit division of -2147483648 by -1.

See the description of the ``nof`` keyword below for division by zero and 
overflow.

If the ``exact`` keyword is present, the result value of the ``sdiv`` is

a :ref:`poison value <poisonvalues>` if the result would be rounded.

``nof``stands for “No Overflow”. If the ``nof`` keyword is present, the 
result is undefined behavior if overflow occurs. This may be result of 
division by zero or dividing the smallest representable integer of the type 
by -1.

If the ``nof`` keyword is not present, the overflow cases described above 
result in poison value.

For vectors, if any element of the division causes overflow, the behavior is 
same as for scalar division with overflow.

Example:

""""""""

.. code-block:: text

       <result> = sdiv i32 4, %var          ; yields i32:result = 4 / %var

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