[llvm-dev] [poison] is select-of-select to logic+select allowed?

[Nuno Lopes via llvm-dev]

Thanks David for pointing out a mistake in the table! I implemented most of the semantics shown in the table in Alive to test these things, but the bug still slipped through, sorry..

 

I’ve updated the table:

 

Summary table of what each transformation allows for %z = select %c, %x, %y. Each column is a different alternative of semantics for select:

 


 

UB if %c poison

+ conditional poison

UB if %c poison + poison if either
%x/%y poison

Conditional poison

+ non-det choice if %c poison

Conditional poison + poison if %c poison**

Poison if any of
%c/%x/%y are poison


SimplifyCFG

✓

 

✓

✓

 


Select->control-flow

✓

✓

 

 

 


Select->arithmetic

 

✓

 

partially 

✓


Select removal

✓

✓

✓ 

✓

✓


Select hoist

✓

✓

✓

 

 


Easy movement

 

 

✓

✓

✓

 

 

IMHO, the 3rd and 4th options are the ones that work best.  Instructions with UB are usually a pain (for e.g. hoisting out of loops).

 

An advantage of the 4th option is that can partially do select->arithmetic, while the 3rd can’t.  For example, this is valid with the 4th option:

%13 = mul nuw i2 %0, -1

%14 = srem i2 %13, -1

%15 = select i1 %1, i2 1, i2 %14

  =>

%15 = zext i1 %1 to i2

 

 

The 4th option, however, can’t do the select->and/or transformations (neither the 3rd can).

 

That said, I’m inclined to choose the 4th option (marked with ** in the table).  That’s the one that the online version of Alive implements BTW.

 

Nuno

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