[llvm-bugs] [Bug 32530] New: inline assembly incompatibility between gcc and clang - mov with offset in intel dialect

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=32530

            Bug ID: 32530
           Summary: inline assembly incompatibility between gcc and clang
                    - mov with offset in intel dialect
           Product: libraries
           Version: trunk
          Hardware: All
                OS: All
            Status: NEW
          Severity: normal
          Priority: P
         Component: Backend: X86
          Assignee: unassignedbugs at nondot.org
          Reporter: superjoe30 at gmail.com
                CC: llvm-bugs at lists.llvm.org

I believe I have found one of these "as-needed" cases for inline assembly:

> LLVM’s support is often implemented on an ‘as-needed’ basis, to support C inline asm code which was supported by GCC. A mismatch in behavior between LLVM and GCC likely indicates a bug in LLVM.

Here is a simple program to print "Hello, world!" on x86_64 Linux:

.intel_syntax noprefix
.text
.globl _start

_start:
    mov rax, 1
    mov rdi, 1
    mov rsi, offset msg
    mov rdx, 14
    syscall

    mov rax, 60
    mov rdi, 0
    syscall

.data
msg:
    .ascii "Hello, world!\n"


GCC:

$ gcc -o hello hello.s -nostdlib && ./hello
Hello, world!


Clang:

$ clang -o hello hello.s -nostdlib && ./hello
hello.s:8:24: error: unknown token in expression
    mov rsi, offset msg
                       ^

I believe the `offset` keyword is the only way to emit a `mov` with the
effective address instead of the dereferenced address.

Related to https://bugs.llvm.org/show_bug.cgi?id=22511
More details: http://stackoverflow.com/questions/43223287/

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