[llvm-bugs] [Bug 50754] New: AArch64 inline asm does not explain why X19 is reserved

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=50754

            Bug ID: 50754
           Summary: AArch64 inline asm does not explain why X19 is
                    reserved
           Product: libraries
           Version: trunk
          Hardware: PC
                OS: Linux
            Status: NEW
          Severity: enhancement
          Priority: P
         Component: Backend: AArch64
          Assignee: unassignedbugs at nondot.org
          Reporter: david.spickett at linaro.org
                CC: arnaud.degrandmaison at arm.com,
                    llvm-bugs at lists.llvm.org, smithp352 at googlemail.com,
                    Ties.Stuij at arm.com

Compiling the following:
extern void fn1(char*);
extern void fn2(int*);

int a;
int main() {
  char b[a];
  fn1(b);
  __asm("nop" : : : "x19");
  int c[64];
  fn2(c);
  return 0;
}

With:
./bin/clang --target=aarch64-linux-gnu -c -O0 test.c

Gives the following warning:
warning: inline asm clobber list contains reserved registers: X19
[-Winline-asm]
  __asm("nop" : : : "x19");
        ^
note: Reserved registers on the clobber list may not be preserved across the
asm statement, and clobbering them may lead to undefined behaviour.
1 warning generated.

The warning is not present when compiling with GCC.

https://godbolt.org/z/3r74Gq6c8

This is because llvm uses X19 as the base pointer for the stack frame. Or at
least, it does when it needs to. So small changes to the code shown can mean
you get a warning or not. (which is correct, but confusing if you're trying to
understand what's going on)

For the frame pointer X29 you get "FP" instead which is more obvious. Even if
you just got "X29" you could look at the ABI docs.

inline asm clobber list contains reserved registers: X19, FP [-Winline-asm]

The base pointer register is not assigned by the ABI it's up to the compiler so
it'd be good to add some explanation to this message.

Perhaps:
note: llvm uses X19 as the stack frame base pointer register.

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