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Clang does the same thing. Probably for compiler-construction and CPU architecture reasons:

• Disentangling that logic into just a swap may allow better optimization in some cases; definitely something it makes sense for a compiler to do early so it can follow values through the swap.

• Xor-swap is total garbage for swapping registers, the only advantage being that it doesn't need a temporary. But xchg reg,reg already does that better.

I'm not surprised that GCC's optimizer recognizes the xor-swap pattern and disentangles it to follow the original values. In general, this makes constant-propagation and value-range optimizations possible through swaps, especially for cases where the swap wasn't conditional on the values of the vars being swapped. This pattern-recognition probably happens soon after transforming the program logic to GIMPLE (SSA) representation, so at that point it will forget that the original source ever used an xor swap, and not think about emitting asm that way.

Hopefully sometimes that lets it then optimize down to only a single mov, or two movs, depending on register allocation for the surrounding code (e.g. if one of the vars can move to a new register, instead of having to end up back in the original locations). And whether both variables are actually used later, or only one. Or if it can fully disentangle an unconditional swap, maybe no mov instructions.

But worst case, three mov instructions needing a temporary register is still better, unless it's running out of registers. I'd guess GCC is not smart enough to use xchg reg,reg instead of spilling something else or saving/restoring another tmp reg, so there might be corner cases where this optimization actually hurts.

(Apparently GCC -Os does have a peephole optimization to use xchg reg,reg instead of 3x mov: PR 92549 was fixed for GCC10. It looks for that quite late, during RTL -> assembly. And yes, it works here: turning your xor-swap into an xchg: https://godbolt.org/z/zs969xh47)

with no memory reads, and the same number of instructions, I don't see any bad impacts and feels odd that it be changed. Clearly there is something I did not think through though, but what is it?

Instruction count is only a rough proxy for one of three things that are relevant for perf analysis: front-end uops, latency, and back-end execution ports. (And machine-code size in bytes: x86 machine-code instructions are variable-length.)

It's the same size in machine-code bytes, and same number of front-end uops, but the critical-path latency is worse: 3 cycles from input a to output a for xor-swap, and 2 from input b to output a, for example.

MOV-swap has at worst 1-cycle and 2-cycle latencies from inputs to outputs, or less with mov-elimination. (Which can also avoid using back-end execution ports, especially relevant for CPUs like IvyBridge and Tiger Lake with a front-end wider than the number of integer ALU ports. And Ice Lake, except Intel disabled mov-elimination on it as an erratum workaround; not sure if it's re-enabled for Tiger Lake or not.)

Also related:

• Why is XCHG reg, reg a 3 micro-op instruction on modern Intel architectures? - and those 3 uops can't benefit from mov-elimination. But on modern AMD xchg reg,reg is only 2 uops.

GCC's real missed optimization here (even with -O3) is that tail-duplication results in about the same static code size, just a couple extra bytes since these are mostly 2-byte instructions. The big win is that the a path then becomes the same length as the other, instead of twice as long to first do a swap and then run the same 3 uops for averaging.

The assembly seems to be checking if either num or den is larger than 2**32 by shifting right by 32 bits and then checking whether the resulting number is 0. Depending on the decision, a 64-bit division (div rsi) or 32-bit division (div esi) is performed.

Something is wrong with your private macro PRINTOUT, or something is wrong with the stetup done before calling the macro in line 6 of your assembler source. I can't tell what it is, because you didn't provide details about that macro (others have suggested to rerun the job with PRINT GEN).

https://godbolt.org/z/PT6T1691W it seems that -O2 -funroll-loops does the trick, apparently that option needs to be on for the pragma to tell GCC how much to unroll. (Update: Or at least makes it have some effect. See comments, this doesn't seem to be a complete answer yet.)

There are two things that make this code look a bit unusual. The first is that it's designed to be accessed by multiple threads without using locks. The second is that it's using a single 64-bit value to store two 32-bit integers.

both the step-out and step-over-instruction requires debug information. You can add debug information with add-symbol-file. If you don't have the debug information, you will have to set a breakpoint or run until the instruction after the call. In this case, that would be one of:

You never pushed a pointer to the second argv string. push %rdx; push %rdi pushes NULL and then the pointer to "/bin/ls", but there is no pointer to your "-laaaaa". You need one more push in between the two. For instance:

When a COPYBOOK is referenced it is selected based on the first dataset where the COPYBOOK is found. The compiler does not look at the dataset name where you are seeing the version number. The version number is a convention to control when new changes are introduced into the environment.