Inefficient bit mask codegen in combination with comparison operators.

Hi. All four test cases should compile to what is essentially a no operation. However the compiler does not omit the unnecessary case_1 and case_2 bit mask operations as it does with case_3 and case_4.

https://play.rust-lang.org/?version=stable&mode=release&edition=2018&gist=5bc8761dc8bebe2fbf7797f47c4a2a26

// Inefficiently compiled.
pub fn case_1(mut u: u32) -> u32 {
    if u <= 0x3F {
        // At this point u is 0..=0x3F and the mask operation does nothing
        u &= 0x7F; // also 0xFF, 0x01FF, ..., 0x7FFF_FFFF.
    }
    u
}

// Inefficiently compiled.
pub fn case_2(mut u: u32) -> u32 {
    if u <= 0x3E {
        // At this point u is 0..=0x3E and the mask operation does nothing
        u &= 0x3F; // also 0x7F, 0xFF, ..., 0x7FFF_FFFF.
    }
    u
}


// Ok
pub fn case_3(mut u: u32) -> u32 {
    if u <= 0x3F {
        // At this point u is 0..=0x3F and the mask operation does nothing
        u &= 0x3F;
    }
    u
}

// Ok
pub fn case_4(mut u: u32) -> u32 {
    if u <= 0x3F {
        // At this point u is 0..=0x3F and the mask operation does nothing
        u &= 0xFFFF_FFFF;
    }
    u
}

Generated assembly:

playground::case_1:
	movl	%edi, %eax
	andl	$127, %eax
	cmpl	$64, %edi
	cmovael	%edi, %eax
	retq

playground::case_2:
	movl	%edi, %eax
	andl	$63, %eax
	cmpl	$63, %edi
	cmovael	%edi, %eax
	retq

playground::case_3:
	movl	%edi, %eax
	retq

playground::case_4:
	movl	%edi, %eax
	retq

Additional example, case_1 should compile as does case_2.

https://play.rust-lang.org/?version=stable&mode=release&edition=2018&gist=0c412cd50128a2cca1539dd299192245

// Inefficiently compiled.
pub fn case_1(mut u: u32) -> u32 {
    if u > 0x7F {
        u = 0x7F;
    }
    // At this point u is 0..=0x7F and the mask operation does nothing.
    u &= 0x7F;
    u
}

pub fn case_2(mut u: u32) -> u32 {
    if u > 0x7F {
        u = 0x7F;
    }
    u
}

playground::case_1:
	cmpl	$127, %edi
	movl	$127, %eax
	cmovbl	%edi, %eax
	andl	$127, %eax
	retq

playground::case_2:
	cmpl	$127, %edi
	movl	$127, %eax
	cmovbl	%edi, %eax
	retq

Clang fails that too but GCC doesn't. Likely to be LLVM missed optimization.
godbolt

Upstream fix for the additional case mentioned in #78745 (comment): llvm/llvm-project@72e0846

Current nightly for Playground A:

playground::case_1:
	movl	%edi, %eax
	andl	$127, %eax
	cmpl	$64, %edi
	cmovael	%edi, %eax
	retq

playground::case_2:
	movl	%edi, %eax
	andl	$63, %eax
	cmpl	$63, %edi
	cmovael	%edi, %eax
	retq

playground::case_3:
	movl	%edi, %eax
	retq

playground::case_4:
	movl	%edi, %eax
	retq

Current nightly for Playground B:

playground::case_1: # @playground::case_1
# %bb.0:
	cmpl	$127, %edi
	movl	$127, %eax
	cmovbl	%edi, %eax
	retq
                                        # -- End function
.set playground::case_2, playground::case_1

So the latter is definitely resolved.

Starting from 1.70, all functions compiled into

        mov     eax, edi
        ret

https://rust.godbolt.org/z/zf8aTxd3v

@rustbot

Same as above, in the current nightly, marking as needing a test.
@rustbot label +E-needs-test

@rustbot

@rustbot claim

Rollup merge of rust-lang#161580 - zakrad:regr-test-78745, r=mu001999

Unrolled build for #161580 in rollup 161638

Rollup merge of #161580 - zakrad:regr-test-78745, r=mu001999

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