Use `SmallVec` to avoid allocations in `from_decimal_string`.

This reduces the number of allocations in a "check clean" build of
`tuple-stress` by 14%, reducing instruction counts by 0.6%.

src/Cargo.lock

@@ -2095,6 +2095,7 @@ version = "0.0.0"
 dependencies = [
  "bitflags 1.0.4 (registry+https://github.com/rust-lang/crates.io-index)",
  "rustc_cratesio_shim 0.0.0",
+ "smallvec 0.6.5 (registry+https://github.com/rust-lang/crates.io-index)",
 ]
 
 [[package]]

src/librustc_apfloat/Cargo.toml

@@ -10,3 +10,4 @@ path = "lib.rs"
 [dependencies]
 bitflags = "1.0"
 rustc_cratesio_shim = { path = "../librustc_cratesio_shim" }
+smallvec = { version = "0.6.5", features = ["union"] }

src/librustc_apfloat/ieee.rs

@@ -11,6 +11,7 @@
 use {Category, ExpInt, IEK_INF, IEK_NAN, IEK_ZERO};
 use {Float, FloatConvert, ParseError, Round, Status, StatusAnd};
 
+use smallvec::{SmallVec, smallvec};
 use std::cmp::{self, Ordering};
 use std::convert::TryFrom;
 use std::fmt::{self, Write};
@@ -1962,7 +1963,7 @@ impl<S: Semantics> IeeeFloat<S> {
         // to hold the full significand, and an extra limb required by
         // tcMultiplyPart.
         let max_limbs = limbs_for_bits(1 + 196 * significand_digits / 59);
-        let mut dec_sig = Vec::with_capacity(max_limbs);
+        let mut dec_sig: SmallVec<[Limb; 1]> = SmallVec::with_capacity(max_limbs);
 
         // Convert to binary efficiently - we do almost all multiplication
         // in a Limb. When this would overflow do we do a single
@@ -2021,11 +2022,11 @@ impl<S: Semantics> IeeeFloat<S> {
 
             const FIRST_EIGHT_POWERS: [Limb; 8] = [1, 5, 25, 125, 625, 3125, 15625, 78125];
 
-            let mut p5_scratch = vec![];
-            let mut p5 = vec![FIRST_EIGHT_POWERS[4]];
+            let mut p5_scratch = smallvec![];
+            let mut p5: SmallVec<[Limb; 1]> = smallvec![FIRST_EIGHT_POWERS[4]];
 
-            let mut r_scratch = vec![];
-            let mut r = vec![FIRST_EIGHT_POWERS[power & 7]];
+            let mut r_scratch = smallvec![];
+            let mut r: SmallVec<[Limb; 1]> = smallvec![FIRST_EIGHT_POWERS[power & 7]];
             power >>= 3;
 
             while power > 0 {
@@ -2064,7 +2065,7 @@ impl<S: Semantics> IeeeFloat<S> {
             let calc_precision = (LIMB_BITS << attempt) - 1;
             attempt += 1;
 
-            let calc_normal_from_limbs = |sig: &mut Vec<Limb>,
+            let calc_normal_from_limbs = |sig: &mut SmallVec<[Limb; 1]>,
                                           limbs: &[Limb]|
              -> StatusAnd<ExpInt> {
                 sig.resize(limbs_for_bits(calc_precision), 0);

src/librustc_apfloat/lib.rs

@@ -53,6 +53,7 @@ extern crate rustc_cratesio_shim;
 
 #[macro_use]
 extern crate bitflags;
+extern crate smallvec;
 
 use std::cmp::Ordering;
 use std::fmt;