On 21/02/13 15:54, Yvan Roux wrote:
Hi,in the example below I want to explicitly generate a "store exclusive pair" instruction with an asm statement: typedef struct { long unsigned int v1; long unsigned int v2; } mtype; int main () { mtype val[2] ; val[0].v1 = 1234; val[0].v2 = 5678; int status; do { __asm__ __volatile__( " stxp %0, %2, %3, %1" : "=&r" (status), "=Q" (val[1]) : "r" (val[0].v1), "r" (val[0].v2) ); } while (status != 0); if (val[1].v1 == 1234 && val[1].v2 == 5678) return 0; return 1; } The generated assembly is: .L7: ldr x0, [sp] ldr x1, [sp,8] .L3: add x3, sp, 16 stxp x2, x0, x1, [x3] cbnz w2, .L7 and the issue is that the assembler is not happy of the register x2 used to store the exclusive access status, it should be w2, but looking at constraint.md it seems that there is no constraint to say that we want the 32bit version of the register. Any idea ?
You may already be aware of this, but like AArch32, the architecture restricts the use of load and store operations that are permitted between LDXP and STXP, which essentially means that any ASM block that uses LDXP must also contain the matching STXP that depends on it. If you don't do this the compiler may introduce random load/store operations (eg spills/reloads) that will kill your exclusive access and make the code unable to proceed.
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