[flang] Fix float-number representation bug

[peixin]

Summary

The float number is represented as (-1)^s * 1.f * 2^(-127) for 32-bit,
where s is the signed flag, f is the mantissa. When the exponent bits
are all zeros, the float number is represented as (-1)^s * 0.f *2^(-126)
for 32-bit, in which case, the intPart is '0'.

[jeanPerier]

There is a unittest that is failing with the buildbots:

/var/lib/buildkite-agent/builds/llvm-project/flang/unittests/Evaluate/real.cpp:43: FAIL: Real4{Integer4{table[j].raw}}.DumpHexadecimal() == table[j].expected
	11
/var/lib/buildkite-agent/builds/llvm-project/flang/unittests/Evaluate/real.cpp:43: FAIL: Real4{Integer4{table[j].raw}}.DumpHexadecimal() == table[j].expected
	12
16839176 tests pass, 2 tests FAIL

[peixin]

Thanks @jeanPerier . Not noticed these test cases are not in fir-dev. The failed test cases are just what this patch fixes. For them, the exponent bits are all zeros. Fix the test cases.

[jeanPerier]

Looks good, thanks

[peixin]

Close this by https://github.com/llvm/llvm-project/commit/0ff322246bcd5c20fdda5e6d1215ee961db2abdc.

flang/lib/Evaluate/real.cpp

if (trailz >= frac.bits - 1) {
         result += frac.SHIFTR(trailz)
                       .IAND(frac.MASKR(lostBits))
                       .SHIFTL(4 - lostBits)
                       .Hexadecimal();
       }
     }
     result += 'p';
     int exponent = Exponent() - exponentBias;
+    if (intPart == '0') {
+      exponent += 1;
+    }
     result += Integer<32>{exponent}.SignedDecimal();
     return result;
   }
 }
 
 template <typename W, int P>
 llvm::raw_ostream &Real<W, P>::AsFortran(
     llvm::raw_ostream &o, int kind, bool minimal) const {

flang/test/Lower/Intrinsics/real.f90

+! RUN: bbc -emit-fir %s -o - | FileCheck %s
+
+! Tests REAL lowering
+subroutine test_real()
+  real(4) :: r4
+  real(8) :: r8
+
+  r4 = real(z'40', kind=4)
+  r8 = real(z'40', kind=8)
+
+end subroutine
+
+! CHECK-LABEL: func @_QPtest_real() {
+! CHECK:  %[[VAL_0:.*]] = fir.alloca f32 {bindc_name = "r4", uniq_name = "_QFtest_realEr4"}
+! CHECK:  %[[VAL_1:.*]] = fir.alloca f64 {bindc_name = "r8", uniq_name = "_QFtest_realEr8"}
+! CHECK:  %[[CST_0:.*]] = arith.constant 8.968310e-44 : f32
+! CHECK:  fir.store %[[CST_0]] to %[[VAL_0]] : !fir.ref<f32>
+! CHECK:  %[[CST_1:.*]] = arith.constant 3.162020e-322 : f64
+! CHECK:  fir.store %[[CST_1]] to %[[VAL_1]] : !fir.ref<f64>
+! CHECK:  return

flang/unittests/Evaluate/real.cpp

const char *expected;
       {0x00000000, "0.0"},
       {0x80000000, "-0.0"},
       {0x3f800000, "0x1.0p0"},
       {0xbf800000, "-0x1.0p0"},
       {0x40000000, "0x1.0p1"},
       {0x3f000000, "0x1.0p-1"},
       {0x7f7fffff, "0x1.fffffep127"},
       {0x00800000, "0x1.0p-126"},
-      {0x00400000, "0x0.8p-127"},
-      {0x00000001, "0x0.000002p-127"},
+      {0x00400000, "0x0.8p-126"},
+      {0x00000001, "0x0.000002p-126"},
       {0, nullptr},
   };
   for (int j{0}; table[j].expected != nullptr; ++j) {
     TEST(Real4{Integer4{table[j].raw}}.DumpHexadecimal() == table[j].expected)
     ("%d", j);
   }
 }