[llvm-dev] Inline ASM ARM syntax to load immediate values with integrated assembler

[Emmanuel Blot via llvm-dev]

Hi,

What would be the proper syntax in a C file, using inline assembly, to
load a immediate value into an ARM register using clang & integrated
assembler?

The following syntax is rejected by LLVM:

    // clang -target armv7em-none-eabi -mthumb

    #define CONSTANT 0x1234

    void __attribute__((naked))
    foo(void)
    {
       asm volatile (
          "ldr r0, =%0  \n"
          :
          :
          "X" (CONSTANT)
          :
          "r0"
       );
    }

“X” transforms the constant into a “#1234”, and LLVM does not seem to
accept “=#1234” syntax. I think the proper syntax should be “ldr r0,
=0x1234”, but I’ve been unable to find a constraint string that would
emit the constant w/o the dash prefix.

The “=%0” syntax is accepted by GCC - I’m not sure it is valid,
although I’m still looking for a comprehensive reference document for
such a syntax.

“mov r0, %0” could work for small encodable value, but not for the
whole 32-bit range where the pseudo-instruction “ldr r0, =constant" is
required.

Thanks.