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Worsened debug build codegen in beta #68855

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rodrimati1992 opened this issue Feb 5, 2020 · 7 comments · Fixed by #70259
Closed

Worsened debug build codegen in beta #68855

rodrimati1992 opened this issue Feb 5, 2020 · 7 comments · Fixed by #70259
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A-associated-items Area: Associated items (types, constants & functions) A-const-eval Area: Constant evaluation (MIR interpretation) I-slow Issue: Problems and improvements with respect to performance of generated code. P-medium Medium priority regression-from-stable-to-beta Performance or correctness regression from stable to beta. T-compiler Relevant to the compiler team, which will review and decide on the PR/issue.

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@rodrimati1992
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rodrimati1992 commented Feb 5, 2020

The assembly generated in debug builds (-C opt-level=0) worsened for this code that uses an associated constant from a trait.This seems to be caused by the fact that the function is generic,even though the associated constant doesn't depend on the generic parameters of the function.

The code:
https://play.rust-lang.org/?version=stable&mode=debug&edition=2018&gist=83dfb31158166b5ed7a92ca5dd7bb610

trait NeedsDrop:Sized{
    const NEEDS:bool=std::mem::needs_drop::<Self>();
}

impl<This> NeedsDrop for This{}

pub fn hello<T>(){
    if <bool>::NEEDS {
        panic!()
    }
}

pub fn hi(){
    hello::<()>();
    hello::<Vec<()>>();
}

The entirety of the generated assembly for stable 1.41.0 in a debug build:

playground::hello:
    retq

playground::hello:
    retq

playground::hi:
    pushq   %rax
    callq   *playground::hello@GOTPCREL(%rip)
    callq   *playground::hello@GOTPCREL(%rip)
    popq    %rax
    retq

The assembly generated for the same functions by rustc 1.42.0-beta.2 (2020-02-04 3d2613e) in a debug build:

playground::hello:
    pushq   %rax
    xorl    %eax, %eax
    testb   $1, %al
    jne .LBB45_2
    jmp .LBB45_1

.LBB45_1:
    popq    %rax
    retq

.LBB45_2:
    leaq    .Lanon.257b63aea292a17f4811b89c35bea688.2(%rip), %rdi
    leaq    .Lanon.257b63aea292a17f4811b89c35bea688.4(%rip), %rdx
    movq    std::panicking::begin_panic@GOTPCREL(%rip), %rax
    movl    $14, %esi
    callq   *%rax
    ud2

playground::hello:
    pushq   %rax
    xorl    %eax, %eax
    testb   $1, %al
    jne .LBB46_2
    jmp .LBB46_1

.LBB46_1:
    popq    %rax
    retq

.LBB46_2:
    leaq    .Lanon.257b63aea292a17f4811b89c35bea688.2(%rip), %rdi
    leaq    .Lanon.257b63aea292a17f4811b89c35bea688.4(%rip), %rdx
    movq    std::panicking::begin_panic@GOTPCREL(%rip), %rax
    movl    $14, %esi
    callq   *%rax
    ud2

playground::hi:
    pushq   %rax
    callq   *playground::hello@GOTPCREL(%rip)
    callq   *playground::hello@GOTPCREL(%rip)
    popq    %rax
    retq

@jonas-schievink jonas-schievink added I-slow Issue: Problems and improvements with respect to performance of generated code. regression-from-stable-to-beta Performance or correctness regression from stable to beta. T-compiler Relevant to the compiler team, which will review and decide on the PR/issue. labels Feb 5, 2020
@pnkfelix
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pnkfelix commented Feb 6, 2020

@rodrimati1992 out of curiosity, what prompted you to investigate this specific code sequence?

(I'm trying to evaluate the severity of this code-quality regression, and it might help for me to get some idea of how wide-spread the problem might be. There are obviously a couple different features interacting in the example you have provided...)

@pnkfelix
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pnkfelix commented Feb 6, 2020

triage: leaving unprioritized. Nominating for discussion at today's T-compiler triage meeting.

@rodrimati1992
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@pnkfelix
I was experimenting to figure out the best design for a feature I want to add to structural eventually,being efficient in debug builds is a plus(but not a hard requirement for me).
The reason I reported this is that if it optimizes fine in non-generic functions,it should also do the same optimization in generic functions(so long as the constant doesn't depend on generic parameters).

@eddyb
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eddyb commented Feb 8, 2020

    xorl    %eax, %eax
    testb   $1, %al

How does LLVM even generate such a code sequence? Well...:

br i1 false, label %bb2, label %bb1, !dbg !945

Okay so I guess LLVM doesn't even simplify the CFG when optimizations are off?


Anyway, the MIR differs. This is a constant folding regression.
cc @rust-lang/wg-mir-opt

@oli-obk
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oli-obk commented Feb 10, 2020

I'm guessing this is caused by #67631

We're in a generic context and resolve has to be conservative so it doesn't accidentally try to inline a default value if it could get a different value in an impl. In this situation that can't happen, but I don't know how to coax resolve to know this.

@eddyb
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eddyb commented Feb 10, 2020

We're in a generic context and resolve has to be conservative so it doesn't accidentally try to inline a default value if it could get a different value in an impl. In this situation that can't happen, but I don't know how to coax resolve to know this.

The fix was already implemented though! Instance::resolve already returns None in the problematic cases. Users of Instance::resolve don't need to special-case anything anymore.

Nor do we ever need to use UserFacing just because something might be polymorphic, because the fix is in Instance::resolve already, as it has been in traits::project for associated types.

I left comments here: #67662 (comment)

@pnkfelix
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T-compiler triage: P-medium. Leaving nomination tag on until we actually assign this to someone.

@pnkfelix pnkfelix added P-medium Medium priority A-associated-items Area: Associated items (types, constants & functions) A-const-eval Area: Constant evaluation (MIR interpretation) labels Feb 20, 2020
Centril added a commit to Centril/rust that referenced this issue Mar 24, 2020
Use Reveal::All in MIR optimizations

Resolves some code review feedback in rust-lang#67662.

Fixes rust-lang#68855

r? @eddyb
Centril added a commit to Centril/rust that referenced this issue Mar 24, 2020
Use Reveal::All in MIR optimizations

Resolves some code review feedback in rust-lang#67662.

Fixes rust-lang#68855

r? @eddyb
@bors bors closed this as completed in 0d5b83d Mar 24, 2020
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Labels
A-associated-items Area: Associated items (types, constants & functions) A-const-eval Area: Constant evaluation (MIR interpretation) I-slow Issue: Problems and improvements with respect to performance of generated code. P-medium Medium priority regression-from-stable-to-beta Performance or correctness regression from stable to beta. T-compiler Relevant to the compiler team, which will review and decide on the PR/issue.
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