[llvm-bugs] [Bug 36067] New: Unsupported ASM constraints for RISCV target

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=36067

            Bug ID: 36067
           Summary: Unsupported ASM constraints for RISCV target
           Product: clang
           Version: trunk
          Hardware: PC
                OS: Linux
            Status: NEW
          Severity: enhancement
          Priority: P
         Component: Frontend
          Assignee: unassignedclangbugs at nondot.org
          Reporter: markwinterrowd4 at gmail.com
                CC: llvm-bugs at lists.llvm.org

When compiling the following reproducer with a RISCV-targeting build of
LLVM/Clang revision 322956:

static inline int a_cas(volatile int *p, int t, int s)
{
        int old, tmp;
        __asm__("li %1, 1\n"
                "1:\n\t"
                "lr.w %0, %2\n\t"
                "bne %0, %3, 1f\n\t"
                "sc.w %1, %4, %2\n\t"
                "bnez %1, 1b\n"
                "1:"
                : "=rJ"(old), "+r"(tmp), "+A"(*p)
                : "r"(t), "r"(s));
        return old;
}

int x = 0;

int main() {
  a_cas(&x, 0, 1);
}

as follows:

clang -target riscv64 test.c

I get the following error output:

test.c:11:19: error: invalid output constraint '=rJ' in asm
                : "=rJ"(old), "+r"(tmp), "+A"(*p)
                  ^

The above implementation of a_cas is taken directly from the atomic_a.h header
file in a RISCV-supporting implementation of the musl libc (which you can find
here: https://github.com/lluixhi/musl-riscv/blob/master/arch/riscv64/atomic_a.h
). Note that J is a valid constraint for the RISCV machine; you can find a
description of its behavior in this GCC doc:

https://gcc.gnu.org/onlinedocs/gcc-7.1.0/gcc/Machine-Constraints.html

This lack of support for RISCV-provincial ASM constraints is not limited to J.
If you replace the "=rJ" with "=r", the following error results:

test.c:11:41: error: invalid output constraint '+A' in asm
                : "=r"(old), "+r"(tmp), "+A"(*p)
                                        ^

indicating that the +A constraint is also not being recognized.

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