[FPEnv] Use single enum to represent rounding mode

Now compiler defines 5 sets of constants to represent rounding mode.
These are:

1. `llvm::APFloatBase::roundingMode`. It specifies all 5 rounding modes
defined by IEEE-754 and is used in `APFloat` implementation.

2. `clang::LangOptions::FPRoundingModeKind`. It specifies 4 of 5 IEEE-754
rounding modes and a special value for dynamic rounding mode. It is used
in clang frontend.

3. `llvm::fp::RoundingMode`. Defines the same values as
`clang::LangOptions::FPRoundingModeKind` but in different order. It is
used to specify rounding mode in in IR and functions that operate IR.

4. Rounding mode representation used by `FLT_ROUNDS` (C11, 5.2.4.2.2p7).
Besides constants for rounding mode it also uses a special value to
indicate error. It is convenient to use in intrinsic functions, as it
represents platform-independent representation for rounding mode. In this
role it is used in some pending patches.

5. Values like `FE_DOWNWARD` and other, which specify rounding mode in
library calls `fesetround` and `fegetround`. Often they represent bits
of some control register, so they are target-dependent. The same names
(not values) and a special name `FE_DYNAMIC` are used in
`#pragma STDC FENV_ROUND`.

The first 4 sets of constants are target independent and could have the
same numerical representation. It would simplify conversion between the
representations. Also now `clang::LangOptions::FPRoundingModeKind` and
`llvm::fp::RoundingMode` do not contain the value for IEEE-754 rounding
direction `roundTiesToAway`, although it is supported natively on
some targets.

This change defines all the rounding mode type via one `llvm::RoundingMode`,
which also contains rounding mode for IEEE rounding direction `roundTiesToAway`.

Differential Revision: https://reviews.llvm.org/D77379

1 files changed, 9 insertions, 0 deletions

diff --git a/llvm/lib/Support/APFloat.cpp b/llvm/lib/Support/APFloat.cpp
index 49f9cf8a..63114fa 100644
--- a/llvm/lib/Support/APFloat.cpp
+++ b/llvm/lib/Support/APFloat.cpp
@@ -171,6 +171,12 @@ namespace llvm {
     return semPPCDoubleDouble;
   }
 
+  constexpr RoundingMode APFloatBase::rmNearestTiesToEven;
+  constexpr RoundingMode APFloatBase::rmTowardPositive;
+  constexpr RoundingMode APFloatBase::rmTowardNegative;
+  constexpr RoundingMode APFloatBase::rmTowardZero;
+  constexpr RoundingMode APFloatBase::rmNearestTiesToAway;
+
   /* A tight upper bound on number of parts required to hold the value
      pow(5, power) is
 
@@ -1323,6 +1329,9 @@ bool IEEEFloat::roundAwayFromZero(roundingMode rounding_mode,
 
   case rmTowardNegative:
     return sign;
+
+  default:
+    break;
   }
   llvm_unreachable("Invalid rounding mode found");
 }