[llvm] [APFloat] Generalize bitcast to also cover x87 semantics (PR #209781)

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<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-llvm-support

Author: janr-bay

<details>
<summary>Changes</summary>

This removes the special case code for bitcasting APInt to a float with x87 semantics in convertF80LongDoubleAPFloatToAPInt. This makes it consistent with previous removal of special cases and will remove one more small obstacle to making APFloat extensible.

---
Full diff: https://github.com/llvm/llvm-project/pull/209781.diff


1 Files Affected:

- (modified) llvm/lib/Support/APFloat.cpp (+12-32) 


``````````diff
diff --git a/llvm/lib/Support/APFloat.cpp b/llvm/lib/Support/APFloat.cpp
index 74f7803c43e67..25e8afd9f9537 100644
--- a/llvm/lib/Support/APFloat.cpp
+++ b/llvm/lib/Support/APFloat.cpp
@@ -3363,34 +3363,8 @@ hash_code hash_value(const IEEEFloat &Arg) {
 // the actual IEEE respresentations.  We compensate for that here.
 
 APInt IEEEFloat::convertF80LongDoubleAPFloatToAPInt() const {
-  assert(semantics ==
-         (const llvm::fltSemantics *)&APFloatBase::semX87DoubleExtended);
-  assert(partCount()==2);
-
-  uint64_t myexponent, mysignificand;
-
-  if (isFiniteNonZero()) {
-    myexponent = exponent+16383; //bias
-    mysignificand = significandParts()[0];
-    if (myexponent==1 && !(mysignificand & 0x8000000000000000ULL))
-      myexponent = 0;   // denormal
-  } else if (category==fcZero) {
-    myexponent = 0;
-    mysignificand = 0;
-  } else if (category==fcInfinity) {
-    myexponent = 0x7fff;
-    mysignificand = 0x8000000000000000ULL;
-  } else {
-    assert(category == fcNaN && "Unknown category");
-    myexponent = 0x7fff;
-    mysignificand = significandParts()[0];
-  }
-
-  uint64_t words[2];
-  words[0] = mysignificand;
-  words[1] =  ((uint64_t)(sign & 1) << 15) |
-              (myexponent & 0x7fffLL);
-  return APInt(80, words);
+  assert(partCount() == 2);
+  return convertIEEEFloatToAPInt<APFloatBase::semX87DoubleExtended>();
 }
 
 APInt IEEEFloat::convertPPCDoubleDoubleLegacyAPFloatToAPInt() const {
@@ -3442,7 +3416,8 @@ APInt IEEEFloat::convertPPCDoubleDoubleLegacyAPFloatToAPInt() const {
 template <const fltSemantics &S>
 APInt IEEEFloat::convertIEEEFloatToAPInt() const {
   assert(semantics == &S);
-  constexpr unsigned int trailing_significand_bits = S.precision - 1;
+  constexpr unsigned int trailing_significand_bits =
+      S.precision - 1 + S.hasExplicitIntegerBit;
   constexpr int integer_bit_part = trailing_significand_bits / integerPartWidth;
   constexpr integerPart integer_bit =
       integerPart{1} << (trailing_significand_bits % integerPartWidth);
@@ -3476,6 +3451,9 @@ APInt IEEEFloat::convertIEEEFloatToAPInt() const {
       llvm_unreachable("semantics don't support inf!");
     myexponent = ::exponentInf(S) + bias;
     mysignificand.fill(0);
+    if constexpr (S.hasExplicitIntegerBit) {
+      mysignificand[0] = integerPart{1} << (trailing_significand_bits - 1);
+    }
   } else {
     assert(category == fcNaN && "Unknown category!");
     if (S.nonFiniteBehavior == fltNonfiniteBehavior::FiniteOnly)
@@ -3487,9 +3465,11 @@ APInt IEEEFloat::convertIEEEFloatToAPInt() const {
   std::array<uint64_t, (S.sizeInBits + 63) / 64> words;
   auto words_iter =
       std::copy_n(mysignificand.begin(), mysignificand.size(), words.begin());
-  if constexpr (significand_mask != 0 || trailing_significand_bits == 0) {
-    // Clear the integer bit.
-    words[mysignificand.size() - 1] &= significand_mask;
+  if constexpr (!S.hasExplicitIntegerBit) {
+    if constexpr (significand_mask != 0 || trailing_significand_bits == 0) {
+      // Clear the integer bit.
+      words[mysignificand.size() - 1] &= significand_mask;
+    }
   }
   std::fill(words_iter, words.end(), uint64_t{0});
   constexpr size_t last_word = words.size() - 1;

``````````

</details>


https://github.com/llvm/llvm-project/pull/209781