[LLVMdev] Missed optimization opportunity in 3-way integer comparison case

[Yuri]

While looking at what llvm writes for this testcase, I noticed that 
there is one redundant operation in resulting assembly. The second 'cmp' 
operation there is essentially identical to the first one, with reversed 
order of arguments. Therefore, it is not needed.

This testcase is a simple integer comparison routine, similar to what 
qsort would take to sort an integer array.

I think llvm should be taking advantage of the preceding instruction and 
not placing the similar instruction again.

rev.208525, optimization level 3.

Yuri


--- C-style original code ---
int mycmp (int i1, int i2) {
     if (i1<i2) {
       return -1;
     } else if (i1>i2) {
       return 1;
     }
     return 0;
}


--- llvm code ---
define i32 @mycmp(i32, i32) #0 {
lbl0:
   %icmp.ULT = icmp ult i32 %0, %1
   br i1 %icmp.ULT, label %lbl1, label %lbl2

lbl1:
   %merge = phi i32 [ -1, %lbl0 ], [ %., %lbl2 ]
   ret i32 %merge

lbl2:
   %icmp.UGT = icmp ugt i32 %0, %1
   %. = zext i1 %icmp.UGT to i32
   br label %lbl1
}

--- intel assembly ---
0000000000000010 <mycmp>:
   10:   55                      push   %rbp
   11:   48 89 e5                mov    %rsp,%rbp
   14:   b8 ff ff ff ff          mov    $0xffffffff,%eax
   19:   39 f7                   cmp    %esi,%edi
   1b:   72 07                   jb     24 <mycmp+0x14>
   1d:   39 fe                   cmp    %edi,%esi   <==== !!! REDUNDANT 
COMPARISON !!!
   1f:   19 c0                   sbb    %eax,%eax
   21:   83 e0 01                and    $0x1,%eax
   24:   5d                      pop    %rbp
   25:   c3                      retq