Auto merge of #42556 - scottmcm:ctz-nz, r=BurntSushi

Get LLVM to stop generating dead assembly in next_power_of_two

It turns out that LLVM can turn `@llvm.ctlz.i64(_, true)` into `@llvm.ctlz.i64(_, false)` ([`ctlz`](http://llvm.org/docs/LangRef.html#llvm-ctlz-intrinsic)) where valuable, but never does the opposite.  That leads to some silly assembly getting generated in certain cases.

A contrived-but-clear example https://is.gd/VAIKuC:
```rust
fn foo(x:u64) -> u32 {
    if x == 0 { return !0; }
    x.leading_zeros()
}
```
Generates
```asm
	testq	%rdi, %rdi
	je	.LBB0_1
	je	.LBB0_3    ; <-- wha?
	bsrq	%rdi, %rax
	xorq	$63, %rax
	retq
.LBB0_1:
	movl	$-1, %eax
	retq
.LBB0_3:
	movl	$64, %eax  ; <-- dead
	retq
```

I noticed this in `next_power_of_two`, which without this PR generates the following:
```asm
	cmpq	$2, %rcx
	jae	.LBB1_2
	movl	$1, %eax
	retq
.LBB1_2:
	decq	%rcx
	je	.LBB1_3
	bsrq	%rcx, %rcx
	xorq	$63, %rcx
	jmp	.LBB1_5
.LBB1_3:
	movl	$64, %ecx  ; <-- dead
.LBB1_5:
	movq	$-1, %rax
	shrq	%cl, %rax
	incq	%rax
	retq
```

And with this PR becomes
```asm
	cmpq	$2, %rcx
	jae	.LBB0_2
	movl	$1, %eax
	retq
.LBB0_2:
	decq	%rcx
	bsrq	%rcx, %rcx
	xorl	$63, %ecx
	movq	$-1, %rax
	shrq	%cl, %rax
	incq	%rax
	retq
```

src/libcore/intrinsics.rs

@@ -1229,6 +1229,23 @@ pub fn volatile_copy_nonoverlapping_memory<T>(dst: *mut T, src: *const T,
     /// ```
     pub fn ctlz<T>(x: T) -> T;
 
+    /// Like `ctlz`, but extra-unsafe as it returns `undef` when
+    /// given an `x` with value `0`.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// #![feature(core_intrinsics)]
+    ///
+    /// use std::intrinsics::ctlz_nonzero;
+    ///
+    /// let x = 0b0001_1100_u8;
+    /// let num_leading = unsafe { ctlz_nonzero(x) };
+    /// assert_eq!(num_leading, 3);
+    /// ```
+    #[cfg(not(stage0))]
+    pub fn ctlz_nonzero<T>(x: T) -> T;
+
     /// Returns the number of trailing unset bits (zeroes) in an integer type `T`.
     ///
     /// # Examples
@@ -1256,6 +1273,23 @@ pub fn volatile_copy_nonoverlapping_memory<T>(dst: *mut T, src: *const T,
     /// ```
     pub fn cttz<T>(x: T) -> T;
 
+    /// Like `cttz`, but extra-unsafe as it returns `undef` when
+    /// given an `x` with value `0`.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// #![feature(core_intrinsics)]
+    ///
+    /// use std::intrinsics::cttz_nonzero;
+    ///
+    /// let x = 0b0011_1000_u8;
+    /// let num_trailing = unsafe { cttz_nonzero(x) };
+    /// assert_eq!(num_trailing, 3);
+    /// ```
+    #[cfg(not(stage0))]
+    pub fn cttz_nonzero<T>(x: T) -> T;
+
     /// Reverses the bytes in an integer type `T`.
     pub fn bswap<T>(x: T) -> T;
 

src/libcore/num/mod.rs

@@ -1262,6 +1262,7 @@ impl isize {
     ($SelfT:ty, $ActualT:ty, $BITS:expr,
      $ctpop:path,
      $ctlz:path,
+     $ctlz_nonzero:path,
      $cttz:path,
      $bswap:path,
      $add_with_overflow:path,
@@ -2184,6 +2185,7 @@ pub fn is_power_of_two(self) -> bool {
         // This method cannot overflow, as in the `next_power_of_two`
         // overflow cases it instead ends up returning the maximum value
         // of the type, and can return 0 for 0.
+        #[inline]
         fn one_less_than_next_power_of_two(self) -> Self {
             if self <= 1 { return 0; }
 
@@ -2192,7 +2194,7 @@ fn one_less_than_next_power_of_two(self) -> Self {
             // (such as intel pre-haswell) have more efficient ctlz
             // intrinsics when the argument is non-zero.
             let p = self - 1;
-            let z = p.leading_zeros();
+            let z = unsafe { $ctlz_nonzero(p) };
             <$SelfT>::max_value() >> z
         }
 
@@ -2236,11 +2238,17 @@ pub fn checked_next_power_of_two(self) -> Option<Self> {
     }
 }
 
+#[cfg(stage0)]
+unsafe fn ctlz_nonzero<T>(x: T) -> T { intrinsics::ctlz(x) }
+#[cfg(not(stage0))]
+unsafe fn ctlz_nonzero<T>(x: T) -> T { intrinsics::ctlz_nonzero(x) }
+
 #[lang = "u8"]
 impl u8 {
     uint_impl! { u8, u8, 8,
         intrinsics::ctpop,
         intrinsics::ctlz,
+        ctlz_nonzero,
         intrinsics::cttz,
         intrinsics::bswap,
         intrinsics::add_with_overflow,
@@ -2253,6 +2261,7 @@ impl u16 {
     uint_impl! { u16, u16, 16,
         intrinsics::ctpop,
         intrinsics::ctlz,
+        ctlz_nonzero,
         intrinsics::cttz,
         intrinsics::bswap,
         intrinsics::add_with_overflow,
@@ -2265,6 +2274,7 @@ impl u32 {
     uint_impl! { u32, u32, 32,
         intrinsics::ctpop,
         intrinsics::ctlz,
+        ctlz_nonzero,
         intrinsics::cttz,
         intrinsics::bswap,
         intrinsics::add_with_overflow,
@@ -2277,6 +2287,7 @@ impl u64 {
     uint_impl! { u64, u64, 64,
         intrinsics::ctpop,
         intrinsics::ctlz,
+        ctlz_nonzero,
         intrinsics::cttz,
         intrinsics::bswap,
         intrinsics::add_with_overflow,
@@ -2289,6 +2300,7 @@ impl u128 {
     uint_impl! { u128, u128, 128,
         intrinsics::ctpop,
         intrinsics::ctlz,
+        ctlz_nonzero,
         intrinsics::cttz,
         intrinsics::bswap,
         intrinsics::add_with_overflow,
@@ -2302,6 +2314,7 @@ impl usize {
     uint_impl! { usize, u16, 16,
         intrinsics::ctpop,
         intrinsics::ctlz,
+        ctlz_nonzero,
         intrinsics::cttz,
         intrinsics::bswap,
         intrinsics::add_with_overflow,
@@ -2314,6 +2327,7 @@ impl usize {
     uint_impl! { usize, u32, 32,
         intrinsics::ctpop,
         intrinsics::ctlz,
+        ctlz_nonzero,
         intrinsics::cttz,
         intrinsics::bswap,
         intrinsics::add_with_overflow,
@@ -2327,6 +2341,7 @@ impl usize {
     uint_impl! { usize, u64, 64,
         intrinsics::ctpop,
         intrinsics::ctlz,
+        ctlz_nonzero,
         intrinsics::cttz,
         intrinsics::bswap,
         intrinsics::add_with_overflow,

src/librustc_trans/intrinsic.rs

@@ -267,7 +267,7 @@ pub fn trans_intrinsic_call<'a, 'tcx>(bcx: &Builder<'a, 'tcx>,
             };
             bcx.call(expect, &[llargs[0], C_i32(ccx, rw), llargs[1], C_i32(ccx, cache_type)], None)
         },
-        "ctlz" | "cttz" | "ctpop" | "bswap" |
+        "ctlz" | "ctlz_nonzero" | "cttz" | "cttz_nonzero" | "ctpop" | "bswap" |
         "add_with_overflow" | "sub_with_overflow" | "mul_with_overflow" |
         "overflowing_add" | "overflowing_sub" | "overflowing_mul" |
         "unchecked_div" | "unchecked_rem" | "unchecked_shl" | "unchecked_shr" => {
@@ -280,6 +280,12 @@ pub fn trans_intrinsic_call<'a, 'tcx>(bcx: &Builder<'a, 'tcx>,
                             let llfn = ccx.get_intrinsic(&format!("llvm.{}.i{}", name, width));
                             bcx.call(llfn, &[llargs[0], y], None)
                         }
+                        "ctlz_nonzero" | "cttz_nonzero" => {
+                            let y = C_bool(bcx.ccx, true);
+                            let llvm_name = &format!("llvm.{}.i{}", &name[..4], width);
+                            let llfn = ccx.get_intrinsic(llvm_name);
+                            bcx.call(llfn, &[llargs[0], y], None)
+                        }
                         "ctpop" => bcx.call(ccx.get_intrinsic(&format!("llvm.ctpop.i{}", width)),
                                         &llargs, None),
                         "bswap" => {

src/librustc_typeck/check/intrinsic.rs

@@ -272,7 +272,8 @@ pub fn check_intrinsic_type<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>,
             "volatile_store" =>
                 (1, vec![ tcx.mk_mut_ptr(param(0)), param(0) ], tcx.mk_nil()),
 
-            "ctpop" | "ctlz" | "cttz" | "bswap" => (1, vec![param(0)], param(0)),
+            "ctpop" | "ctlz" | "ctlz_nonzero" | "cttz" | "cttz_nonzero" | "bswap" =>
+                (1, vec![param(0)], param(0)),
 
             "add_with_overflow" | "sub_with_overflow"  | "mul_with_overflow" =>
                 (1, vec![param(0), param(0)],

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

@@ -14,7 +14,9 @@ mod rusti {
     extern "rust-intrinsic" {
         pub fn ctpop<T>(x: T) -> T;
         pub fn ctlz<T>(x: T) -> T;
+        pub fn ctlz_nonzero<T>(x: T) -> T;
         pub fn cttz<T>(x: T) -> T;
+        pub fn cttz_nonzero<T>(x: T) -> T;
         pub fn bswap<T>(x: T) -> T;
     }
 }
@@ -68,6 +70,21 @@ pub fn main() {
         assert_eq!(ctlz(100u32), 25); assert_eq!(ctlz(100i32), 25);
         assert_eq!(ctlz(100u64), 57); assert_eq!(ctlz(100i64), 57);
 
+        assert_eq!(ctlz_nonzero(1u8), 7); assert_eq!(ctlz_nonzero(1i8), 7);
+        assert_eq!(ctlz_nonzero(1u16), 15); assert_eq!(ctlz_nonzero(1i16), 15);
+        assert_eq!(ctlz_nonzero(1u32), 31); assert_eq!(ctlz_nonzero(1i32), 31);
+        assert_eq!(ctlz_nonzero(1u64), 63); assert_eq!(ctlz_nonzero(1i64), 63);
+
+        assert_eq!(ctlz_nonzero(10u8), 4); assert_eq!(ctlz_nonzero(10i8), 4);
+        assert_eq!(ctlz_nonzero(10u16), 12); assert_eq!(ctlz_nonzero(10i16), 12);
+        assert_eq!(ctlz_nonzero(10u32), 28); assert_eq!(ctlz_nonzero(10i32), 28);
+        assert_eq!(ctlz_nonzero(10u64), 60); assert_eq!(ctlz_nonzero(10i64), 60);
+
+        assert_eq!(ctlz_nonzero(100u8), 1); assert_eq!(ctlz_nonzero(100i8), 1);
+        assert_eq!(ctlz_nonzero(100u16), 9); assert_eq!(ctlz_nonzero(100i16), 9);
+        assert_eq!(ctlz_nonzero(100u32), 25); assert_eq!(ctlz_nonzero(100i32), 25);
+        assert_eq!(ctlz_nonzero(100u64), 57); assert_eq!(ctlz_nonzero(100i64), 57);
+
         assert_eq!(cttz(-1i8 as u8), 0); assert_eq!(cttz(-1i8), 0);
         assert_eq!(cttz(-1i16 as u16), 0); assert_eq!(cttz(-1i16), 0);
         assert_eq!(cttz(-1i32 as u32), 0); assert_eq!(cttz(-1i32), 0);
@@ -93,6 +110,26 @@ pub fn main() {
         assert_eq!(cttz(100u32), 2); assert_eq!(cttz(100i32), 2);
         assert_eq!(cttz(100u64), 2); assert_eq!(cttz(100i64), 2);
 
+        assert_eq!(cttz_nonzero(-1i8 as u8), 0); assert_eq!(cttz_nonzero(-1i8), 0);
+        assert_eq!(cttz_nonzero(-1i16 as u16), 0); assert_eq!(cttz_nonzero(-1i16), 0);
+        assert_eq!(cttz_nonzero(-1i32 as u32), 0); assert_eq!(cttz_nonzero(-1i32), 0);
+        assert_eq!(cttz_nonzero(-1i64 as u64), 0); assert_eq!(cttz_nonzero(-1i64), 0);
+
+        assert_eq!(cttz_nonzero(1u8), 0); assert_eq!(cttz_nonzero(1i8), 0);
+        assert_eq!(cttz_nonzero(1u16), 0); assert_eq!(cttz_nonzero(1i16), 0);
+        assert_eq!(cttz_nonzero(1u32), 0); assert_eq!(cttz_nonzero(1i32), 0);
+        assert_eq!(cttz_nonzero(1u64), 0); assert_eq!(cttz_nonzero(1i64), 0);
+
+        assert_eq!(cttz_nonzero(10u8), 1); assert_eq!(cttz_nonzero(10i8), 1);
+        assert_eq!(cttz_nonzero(10u16), 1); assert_eq!(cttz_nonzero(10i16), 1);
+        assert_eq!(cttz_nonzero(10u32), 1); assert_eq!(cttz_nonzero(10i32), 1);
+        assert_eq!(cttz_nonzero(10u64), 1); assert_eq!(cttz_nonzero(10i64), 1);
+
+        assert_eq!(cttz_nonzero(100u8), 2); assert_eq!(cttz_nonzero(100i8), 2);
+        assert_eq!(cttz_nonzero(100u16), 2); assert_eq!(cttz_nonzero(100i16), 2);
+        assert_eq!(cttz_nonzero(100u32), 2); assert_eq!(cttz_nonzero(100i32), 2);
+        assert_eq!(cttz_nonzero(100u64), 2); assert_eq!(cttz_nonzero(100i64), 2);
+
         assert_eq!(bswap(0x0Au8), 0x0A); // no-op
         assert_eq!(bswap(0x0Ai8), 0x0A); // no-op
         assert_eq!(bswap(0x0A0Bu16), 0x0B0A);