[part 4]
Huh? This looks like the intent here is to avoid the ED instruction, and instead to what the ED would have done in code. So what does ED do? It basically turns a number like this: PIC 9(6)V99 PACKED DECMAL into PIC ZZZ,ZZ9.99. However, our case was simpler. The mask was just ZZ9, so all we really need to do is replace the leading zeros with spaces. And there are only 3 possibilities: * There are no leading zeros, so do nothing * There's 1 leading zero, so move one space * There's 2 leading zeros, so move 2 spaces. So I think what this code is doing, somehow, is figuring out how many spaces need to be moved, and then setting up the display field as one space, followed by the number. Then it does an overlapping move (which we call byte progression) of that space, to the right for a length of x, where x is the number of zeros to overlay. But there's a lot I don't understand here. It may be that what we're seeing is where it generates code for the more complicated situation (where you have a more complex mask) but then because we only have 3 digits and no special mask characters, it optimizes away what it doesn't need. So we see some remnants of the full edit masks even though they're never used. This also shows that there's a trade-off here. It is possible that this really is the absolute most efficient way to do this on a z13 machine. But it isn't what I'd write if I were coding it in assembler, because it is less understandable. Just like we know that the optimizer may inline code repeatedly instead of calling a common section, or do loop unrolling, or other things that are more efficient but less maintainable if you were coding it that way in assembler. Anyway, it looks like the z14 version (arch level 12) is identical to z13, except that: CVD R2,994(,R13) OI 1001(,R13),X'0F' is replaced by: VCVD VRF16,R2,0x3,2 VSTRL VRF16,1033(,R13),0x1 The z14 has Vector Packed Decimal registers, which are a different set of registers than the floating point registers used by the Decimal Floating Point instructions. VCVD is converting the binary R2 to packed decimal, VSTRL is Vector Store Rightmost with Length. I don't see the point here. It is starting with a register and ending in storage in both cases. Only thing I can think of is that this avoids the OI instruction that is acting on storage. And, why is it still using the decimal floating point instructions to count significant digits? There's a Vector Count Leading Zero Digits instruction!
