[llvm-bugs] [Bug 32608] New: Post RA scheduler incorrectly renames callee saved register

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=32608

            Bug ID: 32608
           Summary: Post RA scheduler incorrectly renames callee saved
                    register
           Product: libraries
           Version: trunk
          Hardware: PC
                OS: Linux
            Status: ASSIGNED
          Severity: enhancement
          Priority: P
         Component: Common Code Generator Code
          Assignee: carrot at google.com
          Reporter: carrot at google.com
                CC: llvm-bugs at lists.llvm.org

Compile the following code with options

-fno-exceptions '-mcpu=power8' -O2 '-std=gnu++11'

class StringPiece {
public:
 StringPiece(const char* data, int len): ptr_(data), length_(len) {}
  int find(const char* s, int pos = 0); 

  const char* ptr_;
  int length_;
};

bool foo(const char* hostname, int hostname_len) {
  if (hostname_len <= 0)
    return false;

  const char* hostname_end = hostname + hostname_len;
  if ((hostname_end[-1] == ' '))
    return false;

  if ((hostname[0] == '.') || (hostname_end[-2] == '.'))
    return false;

  if (hostname_len >= 4 && StringPiece(hostname, hostname_len).find("..", 0) !=
-1) 
    return false;

  return true;
}

LLVM at revision r298955 generates following code for the later part of the
function

        ...
        std 30, 48(1)                   # 8-byte Folded Spill
        li 30, 0
        addi 4, 3, .L.str at toc@l
        addi 3, 1, 32
        bl _ZN11StringPiece4findEPKci
        nop
        li 4, 1
        cmpwi 0, 3, -1
        ld 12, 48(1)                    # 8-byte Folded Reload
        isel 3, 4, 30, 2
        addi 1, 1, 64
        ld 0, 16(1)
        mtlr 0
        blr

The instruction
        ld 12, 48(1)
should be
        ld 30, 48(1)
and moved after isel.

The problem can't be reproduced without patch r298955, but the bug is still
there.

The root cause is in post RA scheduler, llvm tries to break anti/output
dependence with register renaming. But for callee saved register restore
instruction, there is no explicit use of the register, simply rename the load
instruction can't restore the value of the register correctly.

We should not rename the callee saved register restore instruction.

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