[llvm-bugs] [Bug 39603] New: failed to eliminate conditional memops

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=39603

            Bug ID: 39603
           Summary: failed to eliminate conditional memops
           Product: libraries
           Version: trunk
          Hardware: PC
                OS: All
            Status: NEW
          Severity: enhancement
          Priority: P
         Component: Scalar Optimizations
          Assignee: unassignedb...@nondot.org
          Reporter: spatel+l...@rotateright.com
                CC: llvm-bugs@lists.llvm.org

This probably doesn't do anything for bug 39597, but we're missing some
fundamental transforms before we get to anything more complicated:

void always_store(int* x, int exchangeVal) {
  *x = (*x != exchangeVal) ? exchangeVal : *x;
}

void never_store(int* x, int exchangeVal) {
  *x = (*x == exchangeVal) ? exchangeVal : *x;
}

void conditional_always_store(int* x, int exchangeVal) {
  if (*x != exchangeVal)
    *x = exchangeVal;
}

void conditional_never_store(int* x, int exchangeVal) {
  if (*x == exchangeVal)
    *x = exchangeVal;
}

https://godbolt.org/z/En099n

As optimized IR:

define void @always_store(i32* nocapture %x, i32 %exchangeVal) {
  store i32 %exchangeVal, i32* %x, align 4
  ret void
}


define void @never_store(i32* nocapture %x, i32 %exchangeVal) {
  ret void
}

define void @conditional_always_store(i32* nocapture %x, i32 %exchangeVal) {
entry:
  %0 = load i32, i32* %x, align 4
  %cmp = icmp eq i32 %0, %exchangeVal
  br i1 %cmp, label %if.end, label %if.then

if.then:                              
  store i32 %exchangeVal, i32* %x, align 4
  br label %if.end

if.end:                      
  ret void
}

define void @conditional_never_store(i32* nocapture %x, i32 %exchangeVal) {
entry:
  %0 = load i32, i32* %x, align 4
  %cmp = icmp eq i32 %0, %exchangeVal
  br i1 %cmp, label %if.then, label %if.end

if.then:       
  store i32 %exchangeVal, i32* %x, align 4
  br label %if.end

if.end:                   
  ret void
}

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