[PATCH] D114768: [ADT][Support] Remove zero-width assertions

[Schuyler Eldridge via Phabricator via llvm-commits]

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This diff removes restrictions around zero/sign extension of zero-width `APInt`s.  Specifically, there is a restriction introduced in https://reviews.llvm.org/D109555 that disallows zero extension of zero-width `APInt`s.  Additionally, there are some old asserts inside `MathExtras`, added in https://reviews.llvm.org/D22442, which assert on sign extension of any zero-width number (which are used pervasively inside `APInt` code to implement its sign extension).  This is attempting to better enable the CIRCT project to rely on `APInt` to represent zero-width wires without introducing lots of additional guards to work around `APInt` methods which assert when used.  More information on what specifically this is attempting to solve for CIRCT is here: https://github.com/llvm/circt/issues/2255


Remove assertion that disallows zero-extending a zero-width APInt.  This
check is too restrictive and makes it difficult to use APInt to model
zero-width things, e.g., zero-width wires in the CIRCT project.

Remove assertions inside MathExtras that disallow zero or sign extending
zero-width things.  Define behavior for this such that zero or sign
extending a zero-width thing returns zero.  This behavior is added to
better support zero-width APInts which use MathExtras internally and it
is undesirable to duplicate MathExtras just to support this in APInt.

Signed-off-by: Schuyler Eldridge <schuyler.eldridge at sifive.com>


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D114768

Files:
  llvm/include/llvm/ADT/APInt.h
  llvm/include/llvm/Support/MathExtras.h


Index: llvm/include/llvm/Support/MathExtras.h
===================================================================
--- llvm/include/llvm/Support/MathExtras.h
+++ llvm/include/llvm/Support/MathExtras.h
@@ -757,34 +757,38 @@
 }
 
 /// Sign-extend the number in the bottom B bits of X to a 32-bit integer.
-/// Requires 0 < B <= 32.
+/// Requires 0 <= B <= 32.  Sign extension of a zero-width value returns zero.
 template <unsigned B> constexpr inline int32_t SignExtend32(uint32_t X) {
-  static_assert(B > 0, "Bit width can't be 0.");
   static_assert(B <= 32, "Bit width out of range.");
+  if (!B)
+    return 0;
   return int32_t(X << (32 - B)) >> (32 - B);
 }
 
 /// Sign-extend the number in the bottom B bits of X to a 32-bit integer.
-/// Requires 0 < B <= 32.
+/// Requires 0 <= B <= 32.  Sign extension of a zero-width value returns zero.
 inline int32_t SignExtend32(uint32_t X, unsigned B) {
-  assert(B > 0 && "Bit width can't be 0.");
   assert(B <= 32 && "Bit width out of range.");
+  if (!B)
+    return 0;
   return int32_t(X << (32 - B)) >> (32 - B);
 }
 
 /// Sign-extend the number in the bottom B bits of X to a 64-bit integer.
-/// Requires 0 < B <= 64.
+/// Requires 0 <= B <= 64.  Sign extension of a zero-width value returns zero.
 template <unsigned B> constexpr inline int64_t SignExtend64(uint64_t x) {
-  static_assert(B > 0, "Bit width can't be 0.");
   static_assert(B <= 64, "Bit width out of range.");
+  if (!B)
+    return 0;
   return int64_t(x << (64 - B)) >> (64 - B);
 }
 
 /// Sign-extend the number in the bottom B bits of X to a 64-bit integer.
-/// Requires 0 < B <= 64.
+/// Requires 0 <= B <= 64.  Sign extension of a zero-width value returns zero.
 inline int64_t SignExtend64(uint64_t X, unsigned B) {
-  assert(B > 0 && "Bit width can't be 0.");
   assert(B <= 64 && "Bit width out of range.");
+  if (!B)
+    return 0;
   return int64_t(X << (64 - B)) >> (64 - B);
 }
 
Index: llvm/include/llvm/ADT/APInt.h
===================================================================
--- llvm/include/llvm/ADT/APInt.h
+++ llvm/include/llvm/ADT/APInt.h
@@ -1458,10 +1458,8 @@
   /// uint64_t. The bitwidth must be <= 64 or the value must fit within a
   /// uint64_t. Otherwise an assertion will result.
   uint64_t getZExtValue() const {
-    if (isSingleWord()) {
-      assert(BitWidth && "zero width values not allowed");
+    if (isSingleWord()) 
       return U.VAL;
-    }
     assert(getActiveBits() <= 64 && "Too many bits for uint64_t");
     return U.pVal[0];
   }


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