[LLVMbugs] [Bug 23575] New: [aarch64][inline assembler] Clang should turn add into sub when necessary.

[bugzilla-daemon at llvm.org]

https://llvm.org/bugs/show_bug.cgi?id=23575

            Bug ID: 23575
           Summary: [aarch64][inline assembler] Clang should turn add into
                    sub when necessary.
           Product: clang
           Version: 3.6
          Hardware: PC
                OS: Linux
            Status: NEW
          Severity: normal
          Priority: P
         Component: -New Bugs
          Assignee: unassignedclangbugs at nondot.org
          Reporter: jacob.bramley at arm.com
                CC: llvmbugs at cs.uiuc.edu
    Classification: Unclassified

(Using Clang 3.6 as packaged in Ubuntu 14.04.)

GNU as will assemble `add w0, w0, #-1` into `sub w0, w0, #1`. This feature is
very useful, and indeed it is the only reasonable use for the "J" constraint.

Here's a simple use-case:

----

int add(int a, int b) {
  int result;

  __asm__ __volatile__ (
    "add %w[result], %w[a], %w[b]\n\t"
    : [result]"=&r" (result), [a]"=r" (a)
    : [b]"IJr" (b)
  );

  return result;
}


int test(int a) {
  return add(a, -1);   // Should be inlined as `sub w0, w0, #1`.
}


----

test.cc:7:5: error: expected compatible register, symbol or integer in range
[0, 4095]
    "add %w[result], %w[a], %w[b]\n\t"
    ^
<inline asm>:1:14: note: instantiated into assembly here
        add w0, w8, #-1

----

We found this problem trying to compile Google V8 using Clang.

I think this is related to bug 20456, but it seems to be slightly different.

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