Enable constexpr on BITREVERSE builtin intrinsics (PR47249)

[RKSimon]

Summary

This enables us to use the __builtin_bitreverse 8/16/32/64 intrinsics inside constexpr code.

[erichkeane]

This looks right to me.

[rsmith]

Can you also add documentation to LanguageExtensions.rst? Thanks!

clang/docs/ReleaseNotes.rst

61

There's no such thing as a "constexpr expression". The right term is "constant expressions".

clang/docs/ReleaseNotes.rst

 Improvements to Clang's diagnostics
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
 - ...
 
 Non-comprehensive list of changes in this release
 -------------------------------------------------
 
-- ...
+- The builtin intrinsics ``__builtin_bitreverse8``, ``__builtin_bitreverse``,
+  ``__builtin_bitreverse32`` and ``__builtin_bitreverse64`` may now be used
+  within constexpr expressions.
 
 New Compiler Flags
 ------------------
 
 - ...
 
 - -fpch-codegen and -fpch-debuginfo generate shared code and/or debuginfo
   for contents of a precompiled header in a separate object file. This object

clang/lib/AST/ExprConstant.cpp

case Builtin::BI__builtin_align_down: {
     if (!Src.isInt())
       return Error(E);
     APSInt AlignedVal =
         APSInt(Src.getInt() & ~(Alignment - 1), Src.getInt().isUnsigned());
     assert(AlignedVal.getBitWidth() == Src.getInt().getBitWidth());
     return Success(AlignedVal, E);
   }
 
+  case Builtin::BI__builtin_bitreverse8:
+  case Builtin::BI__builtin_bitreverse16:
+  case Builtin::BI__builtin_bitreverse32:
+  case Builtin::BI__builtin_bitreverse64: {
+    APSInt Val;
+    if (!EvaluateInteger(E->getArg(0), Val, Info))
+      return false;
+
+    return Success(Val.reverseBits(), E);
+  }
+
   case Builtin::BI__builtin_bswap16:
   case Builtin::BI__builtin_bswap32:
   case Builtin::BI__builtin_bswap64: {
     APSInt Val;
     if (!EvaluateInteger(E->getArg(0), Val, Info))
       return false;
 
     return Success(Val.byteSwap(), E);

clang/test/Sema/constant-builtins-2.c

 char parity4[__builtin_parity(0xb823) == 1 ? 1 : -1];
 char parity5[__builtin_parity(0xb824) == 0 ? 1 : -1];
 char parity6[__builtin_parity(0xb825) == 1 ? 1 : -1];
 char parity7[__builtin_parity(0xb826) == 1 ? 1 : -1];
 char parity8[__builtin_parity(~0) == 0 ? 1 : -1];
 char parity9[__builtin_parityl(1L << (BITSIZE(long) - 1)) == 1 ? 1 : -1];
 char parity10[__builtin_parityll(1LL << (BITSIZE(long long) - 1)) == 1 ? 1 : -1];
 
+char bitreverse1[__builtin_bitreverse8(0x01) == 0x80 ? 1 : -1];
+char bitreverse2[__builtin_bitreverse16(0x3C48) == 0x123C ? 1 : -1];
+char bitreverse3[__builtin_bitreverse32(0x12345678) == 0x1E6A2C48 ? 1 : -1];
+char bitreverse4[__builtin_bitreverse64(0x0123456789ABCDEFULL) == 0xF7B3D591E6A2C480 ? 1 : -1];
+
 char ffs1[__builtin_ffs(0) == 0 ? 1 : -1];
 char ffs2[__builtin_ffs(1) == 1 ? 1 : -1];
 char ffs3[__builtin_ffs(0xfbe71) == 1 ? 1 : -1];
 char ffs4[__builtin_ffs(0xfbe70) == 5 ? 1 : -1];
 char ffs5[__builtin_ffs(1U << (BITSIZE(int) - 1)) == BITSIZE(int) ? 1 : -1];
 char ffs6[__builtin_ffsl(0x10L) == 5 ? 1 : -1];
 char ffs7[__builtin_ffsll(0x100LL) == 9 ? 1 : -1];