[libc] Make log2f correctly rounded for all rounding modes when FMA is not available.

[lntue]

Summary

Add to log2f 2 more exceptional cases got when not using fma for polyeval.

[sivachandra]

libc/src/math/generic/log2f.cpp

121

You did not need this as a special case before so why is it required now?

[lntue]

libc/src/math/generic/log2f.cpp

121

These 2 extra cases I got when I intentionally not using FMA for polyeval, to mimic what would happen if FMA is missing. Another implementation option is only consider these 2 cases when FMA is missing with #ifdef.

libc/src/math/generic/log2f.cpp

 
 INLINE_FMA
 LLVM_LIBC_FUNCTION(float, log2f, (float x)) {
   using FPBits = typename fputil::FPBits<float>;
   FPBits xbits(x);
   int m = 0;
 
   // Hard to round value(s).
-  if (FPBits(x).uintval() == 0x3f81d0b5U) {
+  switch (FPBits(x).uintval()) {
+  case 0x3f81d0b5U: {
     int rounding_mode = fputil::get_round();
     if (rounding_mode == FE_DOWNWARD || rounding_mode == FE_TOWARDZERO) {
       return 0x1.4cdc4cp-6f;
     }
+    break;
+  }
+  case 0x3f7e3274U:
+    if (fputil::get_round() == FE_TONEAREST) {
+      return -0x1.4e1d16p-7f;
+    }
+    break;
+  case 0x3f7d57f5U:
+    if (fputil::get_round() == FE_TOWARDZERO) {
+      return -0x1.ed1c32p-7f;
+    }
+    break;
   }
 
   // Exceptional inputs.
   if (xbits.uintval() < FPBits::MIN_NORMAL ||
       xbits.uintval() > FPBits::MAX_NORMAL) {
     if (xbits.is_zero()) {
       return static_cast<float>(FPBits::neg_inf());
     }

libc/test/src/math/log2f_test.cpp

TEST(LlvmLibcLog2fTest, SpecialNumbers) {
   EXPECT_TRUE(FPBits(__llvm_libc::log2f(neg_inf)).is_nan());
   EXPECT_FP_EQ(neg_inf, __llvm_libc::log2f(0.0f));
   EXPECT_FP_EQ(neg_inf, __llvm_libc::log2f(-0.0f));
   EXPECT_TRUE(FPBits(__llvm_libc::log2f(-1.0f)).is_nan());
   EXPECT_FP_EQ(zero, __llvm_libc::log2f(1.0f));
 }
 
 TEST(LlvmLibcLog2fTest, TrickyInputs) {
-  constexpr int N = 9;
-  constexpr uint32_t INPUTS[N] = {0x3f7d57f5U, 0x3f7ed848U, 0x3f7fd6ccU,
-                                  0x3f7fffffU, 0x3f80079bU, 0x3f81d0b5U,
-                                  0x3f82e602U, 0x3f83c98dU, 0x3f8cba39U};
+  constexpr int N = 10;
+  constexpr uint32_t INPUTS[N] = {
+      0x3f7d57f5U, 0x3f7e3274U, 0x3f7ed848U, 0x3f7fd6ccU, 0x3f7fffffU,
+      0x3f80079bU, 0x3f81d0b5U, 0x3f82e602U, 0x3f83c98dU, 0x3f8cba39U};
 
   for (int i = 0; i < N; ++i) {
     float x = float(FPBits(INPUTS[i]));
     EXPECT_MPFR_MATCH_ALL_ROUNDING(mpfr::Operation::Log2, x,
                                    __llvm_libc::log2f(x), 0.5);
   }
 }