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I've been a bit stumped recently on my decomp work. I've not dealt with C in depth nor have I dealt with IDA's pseudocode system very much at all. I understand asm, but because of how IDA merges instructions together in its pseudocode system, I'm having some trouble finding out how this translates.

In my pseudocode I have the following line:

// v1 is a pointer to RDI (stored as a single in pseudocode)
// v7 is a pointer to R8 (stored as a single in pseudocode)
// v10 is XMM1 (single)

v1[1818] = (float)(v10 * (float)(*v7 - v7[1])) + v7[1];

I'm having trouble understanding what the square brackets do. I was thinking that it was an offset (e.g. "set the data at the location of v1 in memory + 1818i64 bytes to...") but the asm doesn't match up with that addr. What's the difference between setting v1 = vs setting v1[1818] =?

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    In C they denote an array access. To correspond with assembly you might have to multiply the index with size of one member of the array.
    – sudhackar
    Sep 26, 2019 at 3:19
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    Ah-ha! Yes, you were right. That was my mistake. I forgot to multiply 1818 by the size of the value (4 in this case, since it's a single). Suppose I'm just too used to the asm usage of [], yeah? I found the line movss dword ptr [rdi+1C68h], xmm1 -- rdi+1C68h is indeed the right location after doing RDI+(4*1818i16).
    – Eti the Spirit
    Sep 26, 2019 at 3:29
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    If your problem is solved, please write a short answer, accept it and close this question.
    – sudhackar
    Sep 26, 2019 at 10:44

1 Answer 1

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I've found out how this works. I had the right idea, but I had made the mistake of assuming it was n bytes ahead, failing to factor in the size of the actual value at hand.

Since the specific type of v1 was a single, this means I had to multiply the 1818 by 4 bytes. Looking at the corresponding ASM, movss dword ptr [rdi+1C68h], xmm1, I can see that rdi+1C68h comes out to v1 (rdi) + 7272i64 or rdi+1C68h - it checks out.

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