In both cases the arguments and results are passed only in registers, as per the respective calling conventions on Windows and GNU/Linux. In the GNU/Linux variant, the xmm1 is used for accumulating the sum. Since it’s a call-clobbered register (a.k.a caller-saved) it’s stored (and restored) in the stack frame of the caller on each call. … Read more
The MSDN documentation includes information about the x64 registers. x64 extends x64’s 8 general-purpose registers to be 64-bit, and adds 8 new 64-bit registers. The 64-bit registers have names beginning with “r”, so for example the 64-bit extension of eax is called rax. The new registers are named r8 through r15. The lower 32 bits, … Read more
TL:DR: int 0x80 works when used correctly, as long as any pointers fit in 32 bits (stack pointers don’t fit). But beware that strace decodes it wrong unless you have a very recent strace + kernel. int 0x80 zeros r8-r11 for reasons, and preserves everything else. Use it exactly like you would in 32-bit code, … Read more
When people refer to 32-bit and 64-bit assembly, they’re talking about which instruction set you’ll use – they’re also sometimes called Ia32 and x64 in the Intel case, which I presume you’re asking about. There is a lot more going on in the 64-bit case, so starting with 32-bit is probably good; you just need … Read more
If you generate code for a little-endian platform, optimizing four-byte memcmp for inequality to a single DWORD comparison is invalid. When memcmp compares individual bytes it goes from low-addressed bytes to high-addressed bytes, regardless of the platform. In order for memcmp to return zero all four bytes must be identical. Hence, the order of comparison … Read more
Do the following. gdb your_program (gdb) set environment LD_PRELOAD ./yourso.so (gdb) start
libstdc++ apparently does not implement P0602 “variant and optional should propagate copy/move triviality”. You can verify this with: static_assert(std::is_trivially_copyable_v<std::optional<int>>); which fails for libstdc++, and passes for libc++ and the MSVC standard library (which really needs a proper name so we don’t have to call it either “The MSVC implementation of the C++ standard library” or … Read more
Just imagine, how the processor would execute a typical spin-wait loop: 1 Spin_Lock: 2 CMP lockvar, 0 ; Check if lock is free 3 JE Get_Lock 4 JMP Spin_Lock 5 Get_Lock: After a few iterations the branch predictor will predict that the conditional branch (3) will never be taken and the pipeline will fill with … Read more
You need to make GCC use the -m32 flag. You could try writing a simple shell script to your $PATH and call it gcc (make sure you don’t overwrite the original gcc, and make sure the new script comes earlier in $PATH, and that it uses the full path to GCC. I think the code … Read more
I am not going to try to answer the question about SIMD speedup, but provide some detailed comments on poor coding in the scalar version that carried over to the vector version, in a way that doesn’t fit in an SO comment. This code in Intersect(Circle) is just absurd: // Step 3: compute the substitutions, … Read more