[llvm-bugs] [Bug 52541] New: Common code hoisting

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=52541

            Bug ID: 52541
           Summary: Common code hoisting
           Product: libraries
           Version: trunk
          Hardware: PC
                OS: Windows NT
            Status: NEW
          Severity: enhancement
          Priority: P
         Component: Scalar Optimizations
          Assignee: unassignedbugs at nondot.org
          Reporter: llvm at rifkin.dev
                CC: llvm-bugs at lists.llvm.org

The common call to foo() in this below code is hoisted above the if-else.

void A(int x) {
    if(x < 0) { // arbitrary condition...
        foo();
        bar();
    } else {
        foo();
        baz();
    }
}

This transformation is missed in the following code:

void B(int x) {
    if(x < 0) { // arbitrary condition...
        foo();
        bar();
    } else if(x % 2 == 0) { // arbitrary condition...
        foo();
        baz();
    } else {
        foo();
        boz();
    }
}

Which is only transformed to

void B(int x) {
    if(x < 0) { // arbitrary condition...
        foo();
        bar();
    } else {
        foo();
        if(x % 2 == 0) { // arbitrary condition...
            baz();
        } else {
            boz();
        }
    }
}

https://godbolt.org/z/qjjMYWb8o

This is also missed on switches.

This isn't just missed on function calls, it is missed with arithmetic as well:

void B(int x, int y) {
    if(x < 0) { // arbitrary condition...
        foo(x / y);
    } else if(x % 2 == 0) { // arbitrary condition...
        bar(x / y);
    } else {
        baz(x / y);
    }
}

Transforms to:

void B(int x, int y) {
    if(x < 0) { // arbitrary condition...
        int v = x / y;
        foo(v);
    } else {
        int v = x / y;
        if(x % 2 == 0) { // arbitrary condition...
            bar(v);
        } else {
            baz(v);
        }
    }
}

https://godbolt.org/z/vTYj75cx6

It seems there is room for improvement and generalization in this
transformation.

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