rollup merge of #24846: dotdash/fast_cttz8

Currently, LLVM lowers a cttz8 on x86_64 to these instructions:

```asm
    movzbl      %dil, %eax
    bsfl        %eax, %eax
    movl        $32, %ecx
    cmovnel     %eax, %ecx
    cmpl        $32, %ecx
    movl        $8, %eax
    cmovnel     %ecx, %eax
```

To improve the codegen, we can zero extend the 8 bit integer, then set
bit 8 and perform a cttz operation on the extended value. That way
there's no conditional operation involved at all.

This was discovered by  this benchmark: https://github.com/Kimundi/long_strings_without_repeats

Timings on my box with the current nightly:
```
running 4 tests
test bench_cpp_naive_big     ... bench:   5479222 ns/iter (+/- 254222)
test bench_noop_big          ... bench:    571405 ns/iter (+/- 111950)
test bench_rust_naive_big    ... bench:   7798102 ns/iter (+/- 148841)
test bench_rust_unsafe_big   ... bench:   6606488 ns/iter (+/- 67529)
```

Timings with the patch applied:
```
running 4 tests
test bench_cpp_naive_big     ... bench:   5470944 ns/iter (+/- 7109)
test bench_noop_big          ... bench:    568944 ns/iter (+/- 6895)
test bench_rust_naive_big    ... bench:   6795901 ns/iter (+/- 43806)
test bench_rust_unsafe_big   ... bench:   5584879 ns/iter (+/- 5291)
```

Author: alexcrichton

src/libcore/num/mod.rs

@@ -745,7 +745,20 @@ macro_rules! uint_impl {
         #[stable(feature = "rust1", since = "1.0.0")]
         #[inline]
         pub fn trailing_zeros(self) -> u32 {
-            unsafe { $cttz(self as $ActualT) as u32 }
+            // As of LLVM 3.6 the codegen for the zero-safe cttz8 intrinsic
+            // emits two conditional moves on x86_64. By promoting the value to
+            // u16 and setting bit 8, we get better code without any conditional
+            // operations.
+            // FIXME: There's a LLVM patch (http://reviews.llvm.org/D9284)
+            // pending, remove this workaround once LLVM generates better code
+            // for cttz8.
+            unsafe {
+                if $BITS == 8 {
+                    intrinsics::cttz16(self as u16 | 0x100) as u32
+                } else {
+                    $cttz(self as $ActualT) as u32
+                }
+            }
         }
 
         /// Shifts the bits to the left by a specified amount, `n`,

src/test/run-pass/intrinsics-integer.rs

@@ -109,11 +109,6 @@ pub fn main() {
         assert_eq!(cttz32(100), 2);
         assert_eq!(cttz64(100), 2);
 
-        assert_eq!(cttz8(-1), 0);
-        assert_eq!(cttz16(-1), 0);
-        assert_eq!(cttz32(-1), 0);
-        assert_eq!(cttz64(-1), 0);
-
         assert_eq!(bswap16(0x0A0B), 0x0B0A);
         assert_eq!(bswap32(0x0ABBCC0D), 0x0DCCBB0A);
         assert_eq!(bswap64(0x0122334455667708), 0x0877665544332201);