Peter Cordes

Halifax, Canada

GNU/Linux hacker and command line junkie.

(profile pic is from https://xkcd.com/386/)

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Why is the loop instruction slow? Couldn't Intel have implemented it efficiently?

performance assembly x86 intel cpu-architecture asked Mar 2, 2016 at 9:01

stackoverflow.com

75 votes

What happens if you use the 32-bit int 0x80 Linux ABI in 64-bit code?

linux assembly x86-64 system-calls abi asked Sep 7, 2017 at 4:20

stackoverflow.com

53 votes

How exactly do partial registers on Haswell/Skylake perform? Writing AL seems to have a false dependency on RAX, and AH is inconsistent

assembly x86 intel cpu-architecture micro-optimization asked Aug 13, 2017 at 12:05

stackoverflow.com

44 votes

Per-element atomicity of vector load/store and gather/scatter?

x86 atomic sse avx avx512 asked Sep 2, 2017 at 9:56

stackoverflow.com

32 votes

What are the best instruction sequences to generate vector constants on the fly?

assembly x86 sse simd avx asked Jan 29, 2016 at 12:52

stackoverflow.com

22 votes

Does modern PC video hardware support VGA text mode in HW, or does the BIOS emulate it (with System Management Mode)?

performance assembly x86-16 bios vga asked Apr 30, 2020 at 10:57

stackoverflow.com

18 votes

How to convert a binary integer number to a hex string?

assembly x86 hex simd avx512 asked Dec 17, 2018 at 22:14

stackoverflow.com

17 votes

In GNU C inline asm, what are the size-override modifiers for xmm/ymm/zmm for a single operand?

c gcc sse inline-assembly avx512 asked Dec 25, 2015 at 3:35

stackoverflow.com

16 votes

Does undefined behaviour retroactively mean that earlier visible side-effects aren't guaranteed?

c gcc language-lawyer compiler-optimization undefined-behavior asked Sep 20, 2023 at 21:48

stackoverflow.com

15 votes

Vectorizing with unaligned buffers: using VMASKMOVPS: generating a mask from a misalignment count? Or not using that insn at all

gcc assembly x86 sse avx asked Dec 16, 2015 at 8:16

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Top Answers

2123

Why does C++ code for testing the Collatz conjecture run faster than hand-written assembly?

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428

Deoptimizing a program for the pipeline in Intel Sandybridge-family CPUs

stackoverflow.com

355

How much of ‘What Every Programmer Should Know About Memory’ is still valid?

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324

What is the best way to set a register to zero in x86 assembly: xor, mov or and?

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322

Does the C++ standard allow for an uninitialized bool to crash a program?

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263

Is incrementing an int effectively atomic in specific cases?

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182

Why does glibc's strlen need to be so complicated to run quickly?

stackoverflow.com

177

Bubble sort slower with -O3 than -O2 with GCC

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156

Fastest way to do horizontal SSE vector sum (or other reduction)

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149

How to remove "noise" from GCC/clang assembly output?

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129

Why does this delay-loop start to run faster after several iterations with no sleep?

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126

Why does preset "veryfast" in FFmpeg generate the most compressed file compared to all other presets?

superuser.com

104

Where is the x86-64 System V ABI documented?

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104

Possible GCC bug when returning struct from a function

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Why does my browser think that https://1.1.1.1 is secure?

superuser.com

INC instruction vs ADD 1: Does it matter?

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What happens if you use the 32-bit int 0x80 Linux ABI in 64-bit code?

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Argument order to std::min changes compiler output for floating-point

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Floating point division vs floating point multiplication

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Where is the lock for a std::atomic?

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Why is integer assignment on a naturally aligned variable atomic on x86?

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Why are loops always compiled into "do...while" style (tail jump)?

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Why do none of the major compilers optimize this conditional store that checks if the value is already set?

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Boolean values as 8 bit in compilers. Are operations on them inefficient?

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gcc optimization flag -O3 makes code slower than -O2

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32-bit absolute addresses no longer allowed in x86-64 Linux?

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Is there a 128 bit integer in gcc?

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How to get the CPU cycle count in x86_64 from C++?

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Why is this seemingly slower C loop actually twice as fast as the other way?

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Can x86's MOV really be "free"? Why can't I reproduce this at all?

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