Using Uninitialized Memory for Fun and Profit (2008)

• llvm-project

  350 25,563 10.0 C++

  The LLVM Project is a collection of modular and reusable compiler and toolchain technologies.

  

> 1. it's insane. nobody would write such an optimization because there's no possible performance gain, and it changes program behavior

LLVM explicitly has an 'undef' constant value which facilitates this optimization. https://llvm.org/docs/LangRef.html#undefined-values

FWIW, the most important reason compilers do this is to decrease compile time. The compiler notices that some code is insane because it's down an unreachable path, and it deletes the code now instead of waiting to prove the path is unreachable. The later optimizations tend to be slower and scale badly with more code in the function, so deleting it earlier will speed up the compile.

> 2. malloc is just a function, it's not treated in any special way

The compiler is full of optimizations that treat malloc and other functions specially. This file implements an analysis, but the results of the analysis is used by transformations. https://github.com/llvm/llvm-project/blob/main/llvm/lib/Anal...

> 3. there's no guarentee this malloc is a per-spec malloc, it can be a user-defined function for which this is perfectly valid

Yep, there's a flag for that mode, `-ffreestanding` which corresponds to freestanding mode in the C89 standard, chapter 2 section 2.1.2.1.

• glibc

  45 1,213 9.8 C

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Expanding macro gives three GCC function attributes [2]: `__attribute__ ((malloc))`, `__attribute__ ((alloc_size(1)))` and `__attribute__ ((warn_unused_result))`. They are required for GCC (and others recognizing them) to actually ensure that they behave as the standard dictates. Your own malloc-like functions won't be treated same unless you give similar attributes.

[1] https://github.com/bminor/glibc/blob/807690610916df8aef17cd1...

[2] https://gcc.gnu.org/onlinedocs/gcc/Common-Function-Attribute...

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