[llvm] [libc] [libc] Move printf long double to simple calc (PR #75414)

[Guillaume Chatelet via llvm-commits]

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@@ -582,182 +590,259 @@ class FloatToString {
 
   // This takes the index of a block after the decimal point (a negative block)
   // and return if it's sure that all of the digits after it are zero.
-  LIBC_INLINE constexpr bool is_lowest_block(size_t block_index) {
+  LIBC_INLINE constexpr bool is_lowest_block(size_t negative_block_index) {
 #ifdef LIBC_COPT_FLOAT_TO_STR_NO_TABLE
-    return false;
+    // The decimal representation of 2**(-i) will have exactly i digits after
+    // the decimal point.
+    int num_requested_digits =
+        static_cast<int>((negative_block_index + 1) * BLOCK_SIZE);
+
+    return num_requested_digits > -exponent;
 #else
     const int32_t idx = -exponent / IDX_SIZE;
-    const size_t p = POW10_OFFSET_2[idx] + block_index - MIN_BLOCK_2[idx];
+    const size_t p =
+        POW10_OFFSET_2[idx] + negative_block_index - MIN_BLOCK_2[idx];
     // If the remaining digits are all 0, then this is the lowest block.
     return p >= POW10_OFFSET_2[idx + 1];
 #endif
   }
 
   LIBC_INLINE constexpr size_t zero_blocks_after_point() {
 #ifdef LIBC_COPT_FLOAT_TO_STR_NO_TABLE
+    if (exponent < -MANT_WIDTH) {
+      const int pos_exp = -exponent - 1;
+      const uint32_t pos_idx =
+          static_cast<uint32_t>(pos_exp + (IDX_SIZE - 1)) / IDX_SIZE;
+      const int32_t pos_len = ((internal::ceil_log10_pow2(pos_idx * IDX_SIZE) -
+                                internal::ceil_log10_pow2(MANT_WIDTH + 1)) /
+                               BLOCK_SIZE) -
+                              1;
+      const uint32_t len = static_cast<uint32_t>(pos_len > 0 ? pos_len : 0);
+      return len;
+    }
     return 0;
-    // TODO (michaelrj): Find a good algorithm for this that doesn't use a
-    // table.
 #else
     return MIN_BLOCK_2[-exponent / IDX_SIZE];
 #endif
   }
 };
 
-#ifndef LIBC_LONG_DOUBLE_IS_FLOAT64
+#if !defined(LIBC_LONG_DOUBLE_IS_FLOAT64) &&                                   \
+    !defined(LIBC_COPT_FLOAT_TO_STR_NO_SPECIALIZE_LD)
 // --------------------------- LONG DOUBLE FUNCTIONS ---------------------------
 
-template <>
-LIBC_INLINE constexpr size_t FloatToString<long double>::get_positive_blocks() {
-  if (exponent >= -FRACTION_LEN) {
-    const uint32_t idx =
-        exponent < 0
-            ? 0
-            : static_cast<uint32_t>(exponent + (IDX_SIZE - 1)) / IDX_SIZE;
-    const uint32_t len = internal::length_for_num(idx * IDX_SIZE, FRACTION_LEN);
-    return len;
-  } else {
-    return 0;
+template <> class FloatToString<long double> {
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gchatelet wrote:

Not in this patch but ideally `FloatToString` should be templated with the `FPType` enum and then use `FloatToString<get_fp_type<T>()>` to instantiate it. This way you can specialize the algorithm using `FPType::X86_Binary80` which is more precise than `long double`.

https://github.com/llvm/llvm-project/pull/75414