[LLVMbugs] [Bug 22689] New: Two Addr Instruction pass could be improved further

[bugzilla-daemon at llvm.org]

http://llvm.org/bugs/show_bug.cgi?id=22689

            Bug ID: 22689
           Summary: Two Addr Instruction pass could be improved further
           Product: libraries
           Version: trunk
          Hardware: PC
                OS: Linux
            Status: NEW
          Severity: normal
          Priority: P
         Component: Common Code Generator Code
          Assignee: unassignedbugs at nondot.org
          Reporter: wmi at google.com
                CC: llvmbugs at cs.uiuc.edu
    Classification: Unclassified

A problem was identified in TwoAddrInstruction pass and a patch was propsed in
http://reviews.llvm.org/D7806.

However, there are other cases that the patch cannot handle currently. Like the
following one:

1.c:
int M, total;
int A[100], B[100], C[100];

volatile int g;
void goo(int);

void foo() {
  int i, a, b, x, y, z;
  a = 3;
  for (i = 0; i < M; i++) {
    b = i;
    z = g;
    y = b * a;
    x = y + z;
    a = g;
    goo(b);
    total += x;
  }
} 

~/workarea/llvm-r230041/build/bin/clang -O2 -fno-vectorize -fno-unroll-loops -S
1.c

kernel loop in 1.s:
.LBB0_2:                                # %for.body
        movl    g(%rip), %eax
        imull   %ebx, %ebp
        addl    %eax, %ebp
        movl    g(%rip), %r14d
        movl    %ebx, %edi
        callq   goo
        addl    %ebp, total(%rip)
        incl    %ebx
        cmpl    M(%rip), %ebx
        movl    %r14d, %ebp      
        jl      .LBB0_2

The reg copy insn "movl    %r14d, %ebp" could be removed if we switch the
positions of "z = g;" and "y = b * a;" in 1.c

2.c:
int M, total;
int A[100], B[100], C[100];

volatile int g;
void goo(int);

void foo() {
  int i, a, b, x, y, z;
  a = 3;
  for (i = 0; i < M; i++) {
    b = i;
    y = b * a;       // here is the difference with 1.c
    z = g;           // here is the difference with 1.c
    x = y + z;       
    a = g;
    goo(b);
    total += x;
  }
}

~/workarea/llvm-r230041/build/bin/clang -O2 -fno-vectorize -fno-unroll-loops -S
2.c

kernel loop in 2.s:
.LBB0_2:                                # %for.body
        imull   %ebx, %ebp
        movl    g(%rip), %r14d
        addl    %ebp, %r14d
        movl    g(%rip), %ebp
        movl    %ebx, %edi
        callq   goo
        addl    %r14d, total(%rip)
        incl    %ebx
        cmpl    M(%rip), %ebx
        jl      .LBB0_2

The bug is better to be fixed incrementally, so filed this bug to track the
problem.

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