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Preface Overview of the Compiler Getting Started with the Compiler Compiler Features Compiler Coding Practices Compiler Diagnostic Messages Using the Inline and Embedded Assemblers of the AR Compiler support for inline assembly language Inline assembler support in the compiler Restrictions on inline assembler support in the co Inline assembly language syntax with the __asm key Inline assembly language syntax with the asm keywo Inline assembler rules for compiler keywords __asm Restrictions on inline assembly operations in C an Inline assembler register restrictions in C and C+ Inline assembler processor mode restrictions in C Inline assembler Thumb instruction set restriction Inline assembler Vector Floating-Point (VFP) restr Inline assembler instruction restrictions in C and Miscellaneous inline assembler restrictions in C a Inline assembler and register access in C and C++ Inline assembler and the # constant expression spe Inline assembler and instruction expansion in C an Expansion of inline assembler instructions that us Expansion of inline assembler load and store instr Inline assembler effect on processor condition fla Inline assembler expression operands in C and C++ Inline assembler register list operands in C and C Inline assembler intermediate operands in C and C+ Inline assembler function calls and branches in C Inline assembler branches and labels in C and C++ Inline assembler and virtual registers Embedded assembler support in the compiler Embedded assembler syntax in C and C++ Effect of compiler ARM and Thumb states on embedde Restrictions on embedded assembly language functio Compiler generation of embedded assembly language Access to C and C++ compile-time constant expressi Differences between expressions in embedded assemb Manual overload resolution in embedded assembler __offsetof_base keyword for related base classes i Compiler-supported keywords for calling class memb __mcall_is_virtual(D, f) __mcall_is_in_vbase(D, f) __mcall_offsetof_vbase(D, f) __mcall_this_offset(D, f) __vcall_offsetof_vfunc(D, f) Calling nonstatic member functions in embedded ass Calling a nonvirtual member function Calling a virtual member function Accessing sp (r13), lr (r14), and pc (r15) Differences in compiler support for inline and emb Compiler Command-line Options Language Extensions Compiler-specific Features C and C++ Implementation Details What is Semihosting? Via File Syntax Summary Table of GNU Language Extensions Standard C Implementation Definition Standard C++ Implementation Definition C and C++ Compiler Implementation Limits

Inline assembler Thumb instruction set restrictions in C and C++ code

6.10 Inline assembler Thumb instruction set restrictions in C and C++ code

The inline assembler supports Thumb state in ARM architectures v6T2, v6M, and v7. There are a number of Thumb-specific restrictions.

These restrictions are as follows:
  1. TBB, TBH, CBZ, and CBNZ instructions are not supported.
  2. In some cases, the compiler can replace IT blocks with branched code.
  3. The instruction width specifier .N denotes a preference, but not a requirement, to the compiler. This is because, in rare cases, optimizations and register allocation can make it inefficient to generate a 16-bit encoding.
For ARMv6 and lower architectures, the inline assembler does not assemble any Thumb instructions. Instead, on finding inline assembly while in Thumb state, the compiler switches to ARM state automatically. Code that relies on this switch is currently supported, but this practice is deprecated. For ARMv6T2 and higher, the automatic switch from Thumb to ARM state is made if the code is valid ARM assembly but not Thumb.
ARM state can be set deliberately. Inline assembly language can be included in a source file that contains code to be compiled for Thumb in ARMv6 and lower, by enclosing the functions containing inline assembly code between #pragma arm and #pragma thumb statements. For example:
...         // Thumb code
#pragma arm // ARM code. Switch code generation to the ARM instruction set so
            // that the inline assembler is available for Thumb in ARMv6 and lower.
int add(int i, int j)
{
    int res;
    __asm
    {
        ADD   res, i, j   // add here
    }
    return res;
}
#pragma thumb   // Thumb code. Switch back to the Thumb instruction set.
                // The inline assembler is no longer available for Thumb in ARMv6 and
                // lower.
The code must also be compiled using the --apcs /interwork compiler command-line option.
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