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I'm decompiling a game's shaders with RenderDoc in an effort to understand them, and I encountered this segment:

  57:   mul [precise(xyz)]  r8.xyz, r5.yzxy, r6.zxyz
  58:   add [precise(xyz)]  r7.xyz, r7.xyzx, -r8.xyzx
  59:   ishl [precise(x)]  r0.x, r4.z, l(23)
  60:   iadd [precise(x)]  r0.x, r0.x, l(-20769187434139310000000000000000000.000000)
  61:   mov [precise(xz)]  r4.xz, r2.zzwz
  62:   and [precise(xyz)]  r2.xyz, r4.xyzx, l(0x0000ffff, 0x0000ffff, 0x0000ffff, 0)
  63:   utof [precise(xyz)]  r2.xyz, r2.xyzx

And my first thought is, line 60 can't be right, can it? Not only is it a non-integer in an iadd command, the number itself can't fit in an int.

I dug into the actual bytecode to find f8800000. That does indeed convert to the number when read as a float, but this doesn't help me understand why it's being read as a float in the first place. We can see by lines 59 and 62 the decompiler does know how to read things that aren't floats, so that's even weirder.

So I guess what I'm asking is, what am I expecting here for turning this into HLSL? Do I have to do something like asint() or asuint()? Is the negative sign part of the number or not? Every such combination I've tried so far still leads to r0.x being something like 4.16914E+09, and while I don't know if that's the right answer or not, I do know that leaving it alone leads to later variables being NaNs and INFs (which is almost certainly not the right answer).

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1 Answer 1

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It's not working with a massive float. It's the disassembler in RenderDoc that's producing the confusing output.

Looking at the RenderDoc source code, you can see that it just guesses when decoding immediate arguments to decide if it's a float or an integer. It doesn't check what type of value the instruction is expecting - it just decodes them all in the same way regardless of what instruction it is.

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  • \$\begingroup\$ I was just about coming to this same conclusion - elsewhere in the code I was getting "value & 0.0 -> 9", because the "0.0" it was displaying is actually 00ffffff. So if I can't trust RenderDoc, I'll have to do some manual parsing of the raw code. \$\endgroup\$ Commented Jun 13 at 13:39

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