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Compiler User Guide

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

Miscellaneous inline assembler restrictions in C and C++ code

6.13 Miscellaneous inline assembler restrictions in C and C++ code

Compared with armasm or embedded assembly language, the inline assembler has a number of restrictions.

Specifically, these restrictions are as follows:
  • The inline assembler is a high-level assembler, and the code it generates might not always be exactly what you write. Do not use it to generate more efficient code than the compiler generates. Use the embedded assembler or the ARM assembler armasm for this purpose.
  • Some low-level features that are available in the ARM assembler armasm, such as writing to PC, are not supported.
  • Label expressions are not supported.
  • You cannot get the address of the current instruction using dot notation (.) or {PC}.
  • You cannot use the & operator to denote hexadecimal constants. Use the 0x prefix instead. For example:
    __asm { AND x, y, 0xF00 }
  • The notation to specify the actual rotation of an 8-bit constant is not available in inline assembly language. This means that where an 8-bit shifted constant is used, the C flag must be regarded as corrupted if the NZCV flags are updated.
  • You must not modify the stack pointer. This is not necessary because the compiler automatically stacks and restores any working registers as required. The compiler does not permit you to explicitly stack and restore work registers.
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