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Amanieu opened this issue Oct 26, 2015 · 8 comments
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LLVM assertion: Can only indirectify direct input operands! #29382

Amanieu opened this issue Oct 26, 2015 · 8 comments
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A-inline-assembly Area: Inline assembly (`asm!(…)`) C-bug Category: This is a bug. glacier ICE tracked in rust-lang/glacier. I-ICE Issue: The compiler panicked, giving an Internal Compilation Error (ICE) ❄️ P-low Low priority requires-nightly This issue requires a nightly compiler in some way. T-compiler Relevant to the compiler team, which will review and decide on the PR/issue.

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@Amanieu
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Amanieu commented Oct 26, 2015

The following code triggers an LLVM assert:

#![feature(asm)]
unsafe fn test(x: *mut u32) {
    asm!("": "=m" (*x));
}
rustc: /buildslave/rust-buildbot/slave/nightly-dist-rustc-linux/build/src/llvm/lib/CodeGen/SelectionDAG/SelectionDAGBuilder.cpp:6012: void llvm::SelectionDAGBuilder::visitInlineAsm(llvm::ImmutableCallSite): Assertion `(OpInfo.isMultipleAlternative || (OpInfo.Type == InlineAsm::isInput)) && "Can only indirectify direct input operands!"' failed.

One thing of note in the LLVM output is this line:

%5 = call i32 asm "", "=m,~{dirflag},~{fpsr},~{flags}"(), !dbg !199, !srcloc !201

Note that the operand to the asm is left empty for some reason, which is probably wrong.
@steveklabnik steveklabnik added I-ICE Issue: The compiler panicked, giving an Internal Compilation Error (ICE) ❄️ A-inline-assembly Area: Inline assembly (`asm!(…)`) labels Oct 26, 2015
@Amanieu
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Amanieu commented Nov 3, 2015 •
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Here is what clang generates for equivalent code, it seems to translate "m" and "=m" into "*m" and "=*m":

call void asm "", "=*m"(i32* %2) #1, !srcloc !3

Using the =*m constraint directly from rust is difficult because type checking treats it as an output even though the parameter is an input pointer value.

@Amanieu Amanieu mentioned this issue Nov 3, 2015
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bors added a commit that referenced this issue Nov 4, 2015
@bors
Auto merge of #29543 - Amanieu:asm_mem_constraint, r=alexcrichton
effcd29 
The "m" memory constraint in inline assembly is broken (generates incorrect code or triggers LLVM asserts) and should not be used. Instead, indirect memory operands should be used with "\*m", "=\*m" and "+\*m".

Clang does this transparently by transforming "m" constraints into "\*m" indirect constraints, but for now just being able to use "\*m" directly is enough since asm! isn't stable.

While "\*m" works fine as an input operand, "=\*m" and "+\*m" need to be specified as input operands because they take a pointer value as an input. This PR relaxes the constraint checker to allow constraints starting with "=" or "+" if the constraint string contains a "\*", which indicates an indirect operand.

This (indirectly) fixes these issues: #29382, #16383 and #13366. The code will need to be changed to use "\*m" instead of "m".
@Burgos Burgos mentioned this issue Feb 6, 2016
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@brson
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brson commented Apr 11, 2017

Still repros.

@brson brson added T-compiler Relevant to the compiler team, which will review and decide on the PR/issue. P-low Low priority labels Apr 11, 2017
@steveklabnik
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steveklabnik commented Apr 11, 2017 •
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playpen repro: https://is.gd/rUbnON

@eddyb
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eddyb commented Apr 11, 2017

@Amanieu By _ you mean *?

@Amanieu
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Amanieu commented Apr 11, 2017

@eddyb yes

@eddyb
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eddyb commented Apr 11, 2017

@Amanieu We should update the compiler to treat =*m as write to *output, I guess.
Maybe MIR borrowck will be able to handle it better shrug.

@Mark-Simulacrum Mark-Simulacrum added the C-bug Category: This is a bug. label Jul 22, 2017
@istankovic
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Still a problem with rustc 1.26.0-nightly (adf2135ad 2018-03-17).

@Centril Centril added the requires-nightly This issue requires a nightly compiler in some way. label Oct 25, 2019
@rust-lang-glacier-bot rust-lang-glacier-bot added the glacier ICE tracked in rust-lang/glacier. label Apr 29, 2020
@Amanieu
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Amanieu commented May 22, 2020

This issue does not apply to the new asm! (RFC 2850) which doesn't support memory constraints.

The legacy llvm_asm! is deprecated and is no longer maintained.

@Amanieu Amanieu closed this as completed May 22, 2020
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A-inline-assembly Area: Inline assembly (`asm!(…)`) C-bug Category: This is a bug. glacier ICE tracked in rust-lang/glacier. I-ICE Issue: The compiler panicked, giving an Internal Compilation Error (ICE) ❄️ P-low Low priority requires-nightly This issue requires a nightly compiler in some way. T-compiler Relevant to the compiler team, which will review and decide on the PR/issue.
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