`saturating_add` doc is not clear enough

[tesuji]

Godbolt link: https://godbolt.org/z/9rm4jF

So I expect these two functions has the same assembly:

pub fn foo(a: i32, b: i32) -> i32 {
    a.saturating_add(b)
}

pub fn bar(a: i32, b: i32) -> i32 {
    match a.checked_add(b) {
        Some(v) => v,
        None => i32::max_value(),
    }
}

But the result is not:

example::foo:
        xor     eax, eax
        mov     ecx, edi
        add     ecx, esi
        setns   al
        add     eax, 2147483647
        add     edi, esi
        cmovno  eax, edi
        ret

example::bar:
        add     edi, esi
        mov     eax, 2147483647
        cmovno  eax, edi
        ret

[tesuji]

@rustbot modify labels: -C-bug -I-slow T-compiler

[hman523]

Hey I tried solving this but wasn't really sure how to. I believe the implementation to the LLVM generation is right here.

rust/src/librustc_codegen_llvm/intrinsic.rs

Lines 471 to 508 in 22bc9e1

	"saturating_add" | "saturating_sub" => {
	let is_add = name == "saturating_add";
	let lhs = args[0].immediate();
	let rhs = args[1].immediate();
	if llvm_util::get_major_version() >= 8 {
	let llvm_name = &format!("llvm.{}{}.sat.i{}",
	if signed { 's' } else { 'u' },
	if is_add { "add" } else { "sub" },
	width);
	let llfn = self.get_intrinsic(llvm_name);
	self.call(llfn, &[lhs, rhs], None)
	} else {
	let llvm_name = &format!("llvm.{}{}.with.overflow.i{}",
	if signed { 's' } else { 'u' },
	if is_add { "add" } else { "sub" },
	width);
	let llfn = self.get_intrinsic(llvm_name);
	let pair = self.call(llfn, &[lhs, rhs], None);
	let val = self.extract_value(pair, 0);
	let overflow = self.extract_value(pair, 1);
	let llty = self.type_ix(width);
	let limit = if signed {
	let limit_lo = self.const_uint_big(
	llty, (i128::MIN >> (128 - width)) as u128);
	let limit_hi = self.const_uint_big(
	llty, (i128::MAX >> (128 - width)) as u128);
	let neg = self.icmp(
	IntPredicate::IntSLT, val, self.const_uint(llty, 0));
	self.select(neg, limit_hi, limit_lo)
	} else if is_add {
	self.const_uint_big(llty, u128::MAX >> (128 - width))
	} else {
	self.const_uint(llty, 0)
	};
	self.select(overflow, limit, val)
	}
	},

Hope someone can solve this!

[tesuji]

Turn out I was wrong! The saturating_add is working correctly (i.e. those two functions
are not equivalent): https://github.com/rust-lang/rust/pull/58003/files#diff-01076f91a26400b2db49663d787c2576R885-R891

I would add some doctests to show that and close this issue.