[llvm-dev] Questions about inline assembly parameter mapping in llvm IR

[chengeng (C) via llvm-dev]

Dear All,

I compiled the following source code:

typedef unsigned int uint32_t;
typedef unsigned char uint8_t;

int rte_atomic32_cmpset(volatile uint32_t *dst, uint32_t exp, uint32_t src)
{
         uint8_t res;

         asm volatile(
                            "lock ; "
                            "cmpxchgl %[src], %[dst];"
                            "sete %[res];"
                            : [res] "=a" (res),     /* output */
                              [dst] "=m" (*dst)
                            : [src] "r" (src),      /* input */
                              "a" (exp),
                              "m" (*dst)
                            : "memory");            /* no-clobber list */
         return res;
}

And got the following IR:

define dso_local i32 @rte_atomic32_cmpset(i32* %dst, i32 %exp, i32 %src) local_unnamed_addr #0 {
entry:
  %0 = tail call i8 asm sideeffect "lock ; cmpxchgl $2, $1;sete $0;", "={ax},=*m,r,{ax},*m,~{memory},~{dirflag},~{fpsr},~{flags}"(i32* %dst, i32 %src, i32 %exp, i32* %dst) #1, !srcloc !2
  %conv = zext i8 %0 to i32
  ret i32 %conv
}

However, I found the following 3 information does not match:

1.       In the assembler template there are 3 operands (prefixed with $)

2.       In the constrains, there are 2 outputs and 3 inputs

3.       In the parameter list, there are 4 parameters.

Is there some connections between these 3 information? How to interpret them?

Best,
Geng
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