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

[Philip Reames via llvm-dev]

On 12/03/2017 06:57 AM, Nuno Lopes via llvm-dev wrote:
> 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?
Just to comment here, I think this really is worth measuring.  The 
results aren't easy to predict.  Given the optimizer may be able to 
frequently discharge the guard via known-bits or constant-ranges, 
machine-licm can move the divide to the use (thus removing the need for 
the guard), and the vector forms can be done as a ptest/br, the results 
might be nowhere as bad as it might first seem. Particularly not after 
some targeted tuning work.

If someone  wanted to get really fancy, there's also room for implicit 
fault detection and code patching based healing schemes here that I 
don't think have been well explored.  (Or at least, I'm not aware of it.)
>
> 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 thought we generally tried to avoiding emitting target specific 
intrinsics in the vectorizer?
>
> 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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