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LLVM-reserved registers can't be used in inline assembly #85056

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jethrogb opened this issue May 7, 2021 · 3 comments
Open

LLVM-reserved registers can't be used in inline assembly #85056

jethrogb opened this issue May 7, 2021 · 3 comments
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A-inline-assembly Area: inline asm!(..) A-LLVM Area: Code generation parts specific to LLVM. Both correctness bugs and optimization-related issues. F-asm `#![feature(asm)]` (not `llvm_asm`) T-compiler Relevant to the compiler team, which will review and decide on the PR/issue.

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@jethrogb
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jethrogb commented May 7, 2021 •
edited

On several architectures, some registers are considered reserved by LLVM. These registers effectively can't be used by LLVM's inline assembly instruction as in input/output/clobber.

rustc currently disallows using these registers in asm!. This seems to be in contradiction with a stated goal in the RFC:

The intent is that support for asm! should be independent of the rustc back-end used

As such, I'd expect there to be no backend-specific limitations on inline assembly.

Current LLVM-reserved registers are:

  • x86: esi
  • x86-64: rbx
  • ARM: r7
@jethrogb
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jethrogb commented May 7, 2021

At this time, solutions other than “fixing LLVM” seem non-exhaustive or non-trivial. See additional discussion at https://rust-lang.zulipchat.com/#narrow/stream/216763-project-inline-asm/topic/Handling.20clobbers

@jethrogb jethrogb mentioned this issue May 7, 2021
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@jonas-schievink jonas-schievink added A-inline-assembly Area: inline asm!(..) A-LLVM Area: Code generation parts specific to LLVM. Both correctness bugs and optimization-related issues. F-asm `#![feature(asm)]` (not `llvm_asm`) labels May 7, 2021
@bstrie
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bstrie commented May 7, 2021

Un-reserving registers can be done backwards-compatibly, yes? Presumably this does not block stabilization of asm!.

@raoulstrackx raoulstrackx mentioned this issue Jun 7, 2021
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bors bot added a commit to fortanix/rust-sgx that referenced this issue Jun 9, 2021
@bors @raoulstrackx
Merge #336
dbf2bdb 
336: Avoiding use of LLVM reserved register `rbx` r=jethrogb a=raoulstrackx

LLVM reserves register `rbx` in inline assembly (issue: #335). This PR avoids the use of this register until [rust-lang issue 85056](rust-lang/rust#85056) is merged

Co-authored-by: Raoul Strackx <raoul.strackx@fortanix.com>
@Lokathor
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Lokathor commented Jun 24, 2022 •
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It seems that r6 is also reserved by LLVM on ARM with both T32 and A32.

r7 seems to now be available with A32 (but still not T32).

sleffler added a commit to AmbiML/sparrow-cantrip-full that referenced this issue Nov 29, 2022
@sleffler
sel4-sys: fixup MCS-affected syscall wrappers for aarch32
5cb62a9 
- correct seL4_Recv & co for MCS api difference
- split MCS- vs !MCS-syscall wrappers into separate files
- use split inout handling of src => badge

NB: does not compile with MCS because the reply parameter is passed in r6
    which is reserved on some arm targets; see rust-lang/rust#85056.

Bug: 247129956

Change-Id: Idbade9bc50fd7a0968fcbc79a7df69251a473921
GitOrigin-RevId: fe4205dc462031a2910656c3edf2b6811331351d
0xor0ne added a commit to 0xor0ne/debugoff that referenced this issue Dec 26, 2022
@0xor0ne
Fix r7 register usage in ARM syscall
d5c38bd 
In armv7 register r7 is used to hold the syscall number.
However, r7 is also an LLVM reserved register ([1]) and cannot be used
as an input register.
As a temporary workaround we use r8 as temporary input register for
holding the syscall number and then copy r8 into r7 before the actual
system call.

[1] rust-lang/rust#85056
@0xor0ne 0xor0ne mentioned this issue Dec 26, 2022
Closed
@Nilstrieb Nilstrieb added the T-compiler Relevant to the compiler team, which will review and decide on the PR/issue. label Apr 5, 2023
@KristoferHansson KristoferHansson mentioned this issue Feb 6, 2024
Closed
Willmish pushed a commit to Willmish/cantrip that referenced this issue Feb 14, 2024
@sleffler
sel4-sys: fixup MCS-affected syscall wrappers for aarch32
fe4205d 
- correct seL4_Recv & co for MCS api difference
- split MCS- vs !MCS-syscall wrappers into separate files
- use split inout handling of src => badge

NB: does not compile with MCS because the reply parameter is passed in r6
    which is reserved on some arm targets; see rust-lang/rust#85056.

Bug: 247129956

Change-Id: Idbade9bc50fd7a0968fcbc79a7df69251a473921
XuShaohua added a commit to XuShaohua/nc that referenced this issue Feb 17, 2024
@XuShaohua
linux-x86: Disable asm version on x86.
ad2c4ee 
Related: rust-lang/rust#85056

ISSUE: #31
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A-inline-assembly Area: inline asm!(..) A-LLVM Area: Code generation parts specific to LLVM. Both correctness bugs and optimization-related issues. F-asm `#![feature(asm)]` (not `llvm_asm`) T-compiler Relevant to the compiler team, which will review and decide on the PR/issue.
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@bstrie @jethrogb @jonas-schievink @Lokathor @Nilstrieb