[llvm-bugs] [Bug 49525] New: 'P' inline assembly operand modifier should obey -fno-plt

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=49525

            Bug ID: 49525
           Summary: 'P' inline assembly operand modifier should obey
                    -fno-plt
           Product: libraries
           Version: trunk
          Hardware: PC
                OS: Linux
            Status: NEW
          Severity: normal
          Priority: P
         Component: Backend: X86
          Assignee: unassignedbugs at nondot.org
          Reporter: thiago at kde.org
                CC: craig.topper at gmail.com, llvm-bugs at lists.llvm.org,
                    llvm-dev at redking.me.uk, pengfei.wang at intel.com,
                    spatel+llvm at rotateright.com

Unlike GCC, Clang seems not to print the "@PLT" suffix for the 'P' constraint
in PIC mode:

$ cat test.c
extern void f(void); 
void g() { asm("call %P0 # asm" : : "X" (f)); } 
int h() { f(); return 0; }
$ clang -fno-pic -S -o - -O2 test.c | grep call
        callq   f       # asm
        callq   f
$ clang -fPIC -S -o - -O2 test.c | grep call
        callq   f       # asm
        callq   f at PLT
$ gcc -fPIC -S -o - -O2 test.c | grep call
        call f at PLT # asm
        call    f at PLT

That's not a big deal because the linker will generate the PLT anyway if the
function is defined in a shared library being linked.

But the call violates -fno-plt:

$ clang -fno-pic -S -o - -O2 test.c | grep call
        callq   f       # asm
        callq   *f at GOTPCREL(%rip)
$ clang -fPIC -S -o - -O2 test.c | grep call
        callq   f       # asm
        callq   *f at GOTPCREL(%rip)
$ gcc -fPIC -S -o - -O2 test.c | grep call
        call f at PLT # asm
        call    *f at GOTPCREL(%rip)

Equivalent GCC bug report: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=99530

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