[PATCH] D84159: GlobalISel: Handle llvm.roundeven

[Serge Pavlov via Phabricator via llvm-commits]

sepavloff added inline comments.


================
Comment at: llvm/lib/CodeGen/GlobalISel/LegalizerHelper.cpp:2461
+    // operations, round and roundeven are the same operatio.
+    changeOpcode(MI, TargetOpcode::G_INTRINSIC_ROUND);
+    return Legalized;
----------------
I am afraid this change breaks the semantics of `roundeven`. Intrinsic `round` implements the same operation as libm function `round` (http://llvm.org/docs/LangRef.html#id554). In the latest draft (http://www.open-std.org/jtc1/sc22/wg14/www/docs/n2478.pdf, 7.12.9.6p2) the function `round` is described:
```
The round functions round their argument to the nearest integer value in floating-point format,
rounding halfway cases away from zero, regardless of the current rounding direction.
```
In the same draft `roundeven` is described (7.12.9.8p2):
```
The roundeven functions round their argument to the nearest integer value in floating-point format,
rounding halfway cases to even (that is, to the nearest value that is an even integer), regardless of
the current rounding direction.
```
`round` and `roundeven` implement different rounding modes. Both functions do not depend on the current rounding mode.

LIBC variants provide `roundeven` but these implementations look complicated. There must be an algorithm which uses `trunc` and implements `roundeven` suitable for vector operations. Probably the algorithm that uses `remainder` in https://stackoverflow.com/questions/32746523/ieee-754-compliant-round-half-to-even can be used to implement `roundeven` similar to `round` in `LegalizerHelper::lowerIntrinsicRound`.



CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D84159/new/

https://reviews.llvm.org/D84159