[SIL Diagnostics] Fix undefined behavior due to bit shifting in ConstantFolding.cpp

lib/SILOptimizer/Utils/ConstantFolding.cpp

@@ -993,11 +993,14 @@ bool isLossyUnderflow(int srcExponent, uint64_t srcSignificand,
                                   : srcSignificand;
 
   // Compute the significand bits lost due to subnormal form. Note that the
-  // integer part: 1 will use up a significand bit in denormal form.
+  // integer part: 1 will use up a significand bit in subnormal form.
   unsigned additionalLoss = destSem.minExponent - srcExponent + 1;
+  // Lost bits cannot exceed Double.minExponent - Double.significandWidth = 53.
+  // This can happen when truncating from Float80 to Double.
+  assert(additionalLoss <= 53);
 
   // Check whether a set LSB is lost due to subnormal representation.
-  unsigned lostLSBBitMask = (1 << additionalLoss) - 1;
+  uint64_t lostLSBBitMask = ((uint64_t)1 << additionalLoss) - 1;
   return (truncSignificand & lostLSBBitMask);
 }
 

test/SILOptimizer/diagnostic_constant_propagation_floats.swift

@@ -171,6 +171,14 @@ func testHexFloatImprecision() {
   // Smallest non-zero number representable in Double.
   let d2: Double = 0x0.0000000000001p-1022
   _blackHole(d2)
+  let d3: Double = 0x1p-1074
+  _blackHole(d3)
+
+  // Test the case where conversion results in subnormality in the destination.
+  let d4: Float = 0x1p-149
+  _blackHole(d4)
+  let d5: Float = 0x1.8p-149 // expected-warning {{'0x1.8p-149' loses precision during conversion to 'Float}}
+  _blackHole(d5)
 
   // All warnings are disabled during explict conversions.
   _blackHole(Float(0x1.000002p-126))

test/SILOptimizer/diagnostic_constant_propagation_floats_x86.swift

@@ -70,11 +70,24 @@ func testFloatConvertUnderflow() {
   let d4: Double = 5E-324
   _blackHole(d4)
 
+  let e4: Float80 = 0x1p-16445
+  _blackHole(e4)
+
   // FIXME: if a number is so tiny that it underflows even Float80,
   // nothing is reported
   let e1: Float80 = 0x1p-16446
   _blackHole(e1)
 
+  // Test the case where conversion results in subnormality in the destination.
+  let e2: Double = 0x1p-1074
+  _blackHole(e2)
+  // FIXME: there should be a warning in the following line but it is suppressed
+  // conservatively as ConstantFolder thinks that the truncation here is
+  // explicitly requested in the code. A future enhancement to the AST may
+  // produce a warning here.
+  let e3: Double = 0x11p-1074
+  _blackHole(e3)
+
   // All warnings are disabled during explict conversions
   _blackHole(Float(1E-400))
   _blackHole(Double(1E-309))