Tag: io

0 votes
0 answers
69 views
65 votes
4 answers
1.7K views
Why does Windows64 use a different calling convention from all other OSes on x86-64?

AMD has an ABI specification that describes the calling convention to use on x86-64. All OSes follow it, excep

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Rufus
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asked 07 September, 2021
61 votes
8 answers
2.3K views
`testl` eax against eax?

I am trying to understand some assembly. The assembly as follows, I am interested in the testl line: 000319d

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Winford
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asked 07 September, 2021
50 votes
4 answers
832 views
What are CFI directives in Gnu Assembler (GAS) used for?

There seem to be a .CFI directive after every line and also there are wide varities of these ex.,.cfi_startpro

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Royal
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asked 07 September, 2021
57 votes
15 answers
1.1K views
Quickly find whether a value is present in a C array?

I have an embedded application with a time-critical ISR that needs to iterate through an array of size 256 (pr

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Sydney
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asked 07 September, 2021
53 votes
1 answers
1K views
What is the best way to set a register to zero in x86 assembly: xor, mov or and?

All the following instructions do the same thing: set %eax to zero. Which way is optimal (requiring fewest mac

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User demo
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asked 07 September, 2021
63 votes
4 answers
977 views
Why do x86-64 instructions on 32-bit registers zero the upper part of the full 64-bit register?

In the x86-64 Tour of Intel Manuals, I read Perhaps the most surprising fact is that an instruction such a

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Frederick
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asked 07 September, 2021
63 votes
12 answers
3K views
What is the difference between MOV and LEA?

I would like to know what the difference between these instructions is: MOV AX, [TABLE-ADDR] and LEA AX,

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Juliane
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asked 07 September, 2021
54 votes
4 answers
2K views
What are the calling conventions for UNIX & Linux system calls (and user-space functions) on i386 and x86-64

Following links explain x86-32 system call conventions for both UNIX (BSD flavor) & Linux: http://www.int

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Rufus
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asked 07 September, 2021
50 votes
3 answers
1.8K views
Why does GCC generate such radically different assembly for nearly the same C code?

While writing an optimized ftol function I found some very odd behaviour in GCC 4.6.1. Let me show you the cod

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Eddie
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asked 07 September, 2021
66 votes
24 answers
1.4K views
Protecting executable from reverse engineering?

I've been contemplating how to protect my C/C++ code from disassembly and reverse engineering. Normally

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Lashawna
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asked 07 September, 2021
56 votes
4 answers
3K views
Why would introducing useless MOV instructions speed up a tight loop in x86_64 assembly?

Background: While optimizing some Pascal code with embedded assembly language, I noticed an unnecessary MOV i

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Jesenia
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asked 07 September, 2021
62 votes
5 answers
2.5K views
Why does GCC use multiplication by a strange number in implementing integer division?

I've been reading about div and mul assembly operations, and I decided to see them in action by writing a

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Xavier
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asked 07 September, 2021
63 votes
5 answers
1.7K views
Why does Java switch on contiguous ints appear to run faster with added cases?

I am working on some Java code which needs to be highly optimized as it will run in hot functions that are inv

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Kristopher
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asked 07 September, 2021
58 votes
11 answers
1.9K views
Why does C++ code for testing the Collatz conjecture run faster than hand-written assembly?

I wrote these two solutions for Project Euler Q14, in assembly and in C++. They implement identical brute forc

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Deedee
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asked 07 September, 2021
64 votes
11 answers
1.9K views
Replacing a 32-bit loop counter with 64-bit introduces crazy performance deviations with _mm_popcnt_u64 on Intel CPUs

I was looking for the fastest way to popcount large arrays of data. I encountered a very weird effect: Changin

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Daniel
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asked 07 September, 2021
62 votes
14 answers
663 views
Is < faster than <=?

Is if (a < 901) faster than if (a <= 900)? Not exactly as in this simple example, but there are slight p

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Trudy
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asked 07 September, 2021
58 votes
12 answers
935 views
Why doesn&apos;t GCC optimize a*a*a*a*a*a to (a*a*a)*(a*a*a)?

I am doing some numerical optimization on a scientific application. One thing I noticed is that GCC will optim

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Venice
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asked 07 September, 2021
63 votes
15 answers
2.6K views
Android Studio Collapse definitions and methods

How can I collapse all definitions and methods within the Android Studio editor? Visual Studio has that optio

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Ali
0 1 0 9
asked 07 September, 2021