How to avoid generation of extra assembly instruction by gcc compiler for an inline assembly function


I have two questions about generation of assembly code by gcc as a result of using inline assembly function in C file. Let me first explains my situation/code and then I will ask questions.


I have defined following macro NBL2SPM which calls the mySimMagic2() function.

#define NBL2SPM(compactCode,MemAdr)          mySimMagic2(SIM_CMD_USER,compactCode,MemAdr)

mySimMagic2() is defined as below.

#define mySimMagic2(cmd, MemAdr, CompactCode) ({        \
void *_arg0 = (MemAdr);\
unsigned long _arg1 = (CompactCode), _res; \
__asm__ __volatile__ (                    \        
    "\tmovq (%%rdx), %%rbx" "\n"\
    "\txchg %%bx, %%bx\n"                     \
    : "=a" (_res)           /* output    */   \
    : "d"(_arg0),                             \
      "c"(_arg1)            /* input     */   \
    : "%rbx" ); /* clobbered */ \
    _res;                                     \

I use the macro in C-source file in following way


And it results into following assembly code.

    movq    %rax, -400104(%rbp)
    leaq    -400032(%rbp), %rax
    addq    $200000, %rax
    movq    %rax, -400096(%rbp)
    movq    $261, -400088(%rbp)
    movq    -400096(%rbp), %rax
    movq    -400088(%rbp), %rcx
    movq    %rax, %rdx        
    movq    (%rdx), %rbx
    xchg    %bx, %bx

Question 1: Why compiler (gcc) generates assembly instructions (i.e. movq %rax, -400096(%rbp) and movq $261, -400088(%rbp)) to first move the register RAX and immediate value $261 to memory and then loads them back from memory into registers (i.e. movq -400096(%rbp), %rax and movq 400088(%rbp), %rcx)? What is the purpose/advantage of this storing of data to memory and then loading back the same data to registers?

Question 2: Suppose that I want to avoid this "unwanted" saving of register/immediate value to memory and then loading it back to register. Is there a way to force the compiler (gcc) to not generate these "unwanted" memory store/load instructions.

asked on Stack Overflow Dec 29, 2017 by NUM • edited Dec 29, 2017 by Michael Petch

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