[LLVMdev] Regarding codegenprepare transformations

Dimitri Tcaciuc dtcaciuc at gmail.com
Wed Jan 16 23:26:50 PST 2013

Hello everyone,

For the context of question, I have a small loop written in a custom
front-end which can be fairly accurately expressed with the following C

    struct Array {
        double * data;
        long n;

    #define X   0
    #define Y   1
    #define Z   2

    void f(struct Array * restrict d, struct Array * restrict out, const
long n)
        for (long i = 0; i < n; ++ i) {
            for (long j = i + 1; j < n; ++ j) {
                out->data[X] = d->data[i * 3 + X] * d->data[j * 3 + X];
                out->data[Y] = d->data[i * 3 + Y] * d->data[j * 3 + Y];
                out->data[Z] = d->data[i * 3 + Z] * d->data[j * 3 + Z];

I'm looking through the IR transformations during passes added
by LLVMTargetMachine::addPassesToEmitFile and seeing something I could use
some help explaining. The point of interest is between
'unreachableblockelim' and 'codegenprepare' passes. Here is the paste of IR
after each pass


I've annotated 3 spots in the code with stars. In (1), after
unreachableblockelim, addr89 is precomputed outside the loop once and is
used in store in (2). However, in (3), after codegenprepare, there is now a
bunch of math being done every loop iteration to get the address for the
same store. Additionally, looks like the same thing is happening for
several addresses above as well.

Does this look right? Why would those calculations be moved back into the


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