Static Values in Assembler Code

Question

I have the following simple code: ``` #include <cmath> struct init_sin { typedef double type; static constexpr type value(int index) { return 3*std::pow(std::sin(index * 2.0 * 3.1415 / 20.0),1.999); } }; int main(){ static double VALUE = init_sin::value(10); double VALUE_NONSTAT = 3*std::pow(std::sin(10 * 2.0 * 3.1415 / 20.0),1.999); return int(VALUE_NONSTAT); } ``` I would like to find out what the meaning of the assembler code is of this given piece. Here the link to the assembly: http://pastebin.com/211AfSYh I thought that `VALUE` is compile time computed and directly as value in the assembler code Which should be in this line if I am not mistaken: ``` 33 .size main, .-main 34 .data 35 .align 8 36 .type _ZZ4mainE5VALUE, @object GAS LISTING /tmp/ccbPDNK8.s page 2 37 .size _ZZ4mainE5VALUE, 8 38 _ZZ4mainE5VALUE: 39 0000 15143B78 .long 2017137685 40 0004 45E95B3E .long 1046210885 ``` - Why are there two values with `.long` ? And why are the types long? (its a double?, maybe in assemble…

Accepted Answer

`.long` in this assembler syntax implies a 32-bit number. Because a `double` is 64-bits, what you're seeing there is the two 32-bit parts of `VALUE`, in their `double` representation. You'll also notice above it that it's being aligned to an 8-byte boundary (through the `.align` statement) and that it's size is 8 (through the `.size` statement). Also, it's in the main `.data` segment, which is typically used for global-scope variables which are not initialised to zero (as a side-note, `.bss` is typically used to zero-initialised global scope variables).

The `VALUE_NONSTAT` can be seen being loaded into `%rax` here, which is the 64-bit version of the `AX` register:

```
             V
 20 0004 48B81514              movabsq $4493441537811354645, %rax
 20      3B7845E9
 20      5B3E
```

Recalling that `15143B7845E95B3E` is the representation of the value of `3*std::pow(std::sin(index * 2.0 * 3.1415 / 20.0),1.999)` when stored in a `double`, you can see the internal value in hex starting around where I inserted a `V`.

Later statements then push it onto the stack (`movq %rax, -8(%rbp)`), then load it into an FP register (`movsd -8(%rbp), %xmm0`) before converting it to an integer and storing it in `%eax`, which is the register for return values (`cvttsd2si %xmm0, %eax`) and then returning from the routine, using `ret`.

In any case, at the optimisation level you're using (and probably below), your compiler has figured out that `VALUE_NONSTAT` is a constant expression, and just inlined it at compile time instead, since the value is fully known at compile time.

Static Values in Assembler Code

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