[llvm-bugs] [Bug 42189] New: Avoid copy to reg before cmp

via llvm-bugs llvm-bugs at lists.llvm.org
Fri Jun 7 12:11:18 PDT 2019


            Bug ID: 42189
           Summary: Avoid copy to reg before cmp
           Product: libraries
           Version: trunk
          Hardware: PC
                OS: Linux
            Status: NEW
          Severity: enhancement
          Priority: P
         Component: Backend: X86
          Assignee: unassignedbugs at nondot.org
          Reporter: david.bolvansky at gmail.com
                CC: craig.topper at gmail.com, llvm-bugs at lists.llvm.org,
                    llvm-dev at redking.me.uk, spatel+llvm at rotateright.com

int g();
int f();

int shortmax(short c)
  if (c == 32767)
    return g();
  return f();

  movzx eax, di
  cmp eax, 32767
  jne .LBB0_2
  jmp g()
  jmp f()

But shouldn't we just use cmp di, 32767?

And I am just looking what -Os produces, and this is really interesting...

  movzx eax, di
  cmp eax, 32767
  jne f() // neat
  jmp g()

So for this pattern
if (x) call fn1();
else   call fn2();
Possibly -O2/O3 should do this tranformation too?

Maybe ideal code (what -O3 should produce) for my example is:
  cmp di, 32767
  jne f()
  jmp g()


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