How to prevent auto optimization from for loop to memset() in Visual Studio?

Question

I'm developing a program without using CRT , so, some third party using very naive implementation of memset() likes:

char x[10];
for(int i= 0; i< 10; i++) {
  x[i] = '\0';
}

It's alot of this type of code in that 3rd party library, and, I do not like messing around with it, so, how could I prevent Visual Studio from automatically converting from for loop to memset() ?

Edit: Why this is a problem ? Since, my program does not use CRT, so, if Visual Studio auto converts the for loop to memset(), it will cause the error:

Unresolved external symbol _memset

char x[10] = {0}; will do the trick, too, and you would not need the loop at all. — Sergey Kalinichenko, Feb 08 '18 at 02:57
dasblinkenlight: That's the code inside 3rd party lib, and, I do not want to modify it. — Bùi Văn Thủ, Feb 08 '18 at 02:58
coderredoc: I'm not using CRT (the CRT is not linked to my program, so, I could not use it) — Bùi Văn Thủ, Feb 08 '18 at 02:58
@BùiVănThủ.: Compiler optimizes it doesn't mean that it will chance your `.c` files - it's in assembly or machine code. You are aware of that? — user2736738, Feb 08 '18 at 02:59
@coderredoc : Common Language Runtime, i.e. Standard C library, e.g. functions in stdio.h — Bùi Văn Thủ, Feb 08 '18 at 03:02
perhaps compile with debugging enabled? That should force it to preserve everything right? — smac89, Feb 08 '18 at 03:07
quick question, how do you know that VS is converting the code to `memset`? — smac89, Feb 08 '18 at 03:08
You could probably add /Od (disable optimizations) in your project settings. But why not just write a simple memset function yourself and link it in? — Dave, Feb 08 '18 at 03:09
@smac89 : see my edit, I'm not using memset(), and get error of "Unresolved external" — Bùi Văn Thủ, Feb 08 '18 at 03:11
That's kinda hard to give an answer to. Unless you're saying that this loop is all the code you have and VS is some how converting *that* to `_memset`, you will have to provide more evidence that no part of your code is calling off to `memset`. Only then can we focus solely on what you have provided to assist you with an answer. I don't have VS, but I am willing to bet that if I put that code in a VS compiler, it will not call memset — smac89, Feb 08 '18 at 03:14
https://social.msdn.microsoft.com/Forums/vstudio/en-US/29a4236b-f9bc-4558-a87b-6eb5f86989cd/error-lnk2019-unresolved-external-symbol-strncmp-referenced-in-function-public-int-thiscall?forum=vclanguage — smac89, Feb 08 '18 at 03:21
Also see this other question which may be a duplicate of yours: https://stackoverflow.com/questions/34285476/memcmp-linker-error-visual-studio-2015 — smac89, Feb 08 '18 at 03:22
@Dave : implement a memset() myself, and got this error: https://msdn.microsoft.com/en-us/library/b8s7ctwz.aspx — Bùi Văn Thủ, Feb 08 '18 at 03:23
@smac89 Replacing zero-initialization code with `memset` call is a common technique used by _all_ compilers. — , Feb 11 '18 at 03:46

score 0 · Answer 1 · answered Feb 08 '18 at 07:16

The only relevant compiler option I see in the list of compiler options, besides disabling optimization, is /kernel, which produces a binary that can run in the Windows kernel. That is a big hammer; it alters compilation in several ways. Building for the kernel is something one might expect would instruct the compiler not to use library routines. However, the specific page for that switch does not mention it, so I doubt the switch includes that.

Given this, I do not think Visual Studio has a feature of producing stand-alone code that does not use library routines.

Serge Ballesta · Answer 2 · 2018-02-11T10:16:17.417

If I correctly remember, MSVC used to have an option for explicitely not use the C run time library. My version was not English, but it should be something like: Use standard libraries. It was used to produce an executable with no dependancies beyond Windows API, which is your current requirement.

According to the MSVC page about NODEFAULTLIB, the option still exists on recent versions and should do what you need:

If you use /NODEFAULTLIB, for example, to build your program without the C run-time library, you may have to also use /ENTRY to specify the entry point (function) in your program.

I could not test (no access to a MSVC installation), but I would bet a coin that MSVC will not try to use a function from the CRT when you set this option.

IMHO, MSVC should not optimize the loop to a memset call, because it defeats a stand alone environment build. Anyway, a possible workaround is to provide à private memset version wrapping the WinAPI function ZeroMemory.

Unfortunately, even with /NODEFAULTLIB, the compiler still optimizes to memset() — Bùi Văn Thủ, Feb 11 '18 at 04:00

How to prevent auto optimization from for loop to memset() in Visual Studio?

2 Answers2