Why does clang generate rsqrt, if stack-protector is turned on?

Question

Check out this simple code:

#include <cmath>

float foo(float in) {
    return sqrtf(in);
}

With -ffast-math, clang generates sqrtss, as it is expected. But, if I use -fstack-protector-all as well, it changes sqrtss to rsqrtss, as you can see at godbolt. Why?

Answer 1

The short and sweet:

rsqrtss is safer and, as a result, less accurate and slower.

sqrtss is faster and, as a result, less safe.

Why is rsqrtss safer?

• It doesn't use the whole XMM register.

Why is rsqrtss slower?

• Because it needs more registers to perform the same action as sqrtss.

Why does rsqrtss use a reciprocal?

• In a pinch, it seems that the reciprocal of a square root can be calculated faster and with less memory. Pico-spelenda: Lots of math.

The long and bitter:

Research

• What does -ffast-math do?

  -ffast-math
      Enable fast-math mode. This defines the __FAST_MATH__ preprocessor
      macro, and lets the compiler make aggressive, potentially-lossy
      assumptions about floating-point math. These include:
  
      Floating-point math obeys regular algebraic rules for real numbers (e.g. + and * are associative, x/y == x * (1/y), and (a + b) * c == a * c + b * c),
      operands to floating-point operations are not equal to NaN and Inf, and
      +0 and -0 are interchangeable.
  

• What does -fstack-protector-all do?

  • This answer can be found here.

  • Basically, it "forces the usage of stack protectors for all functions".

• What is a "stack protector"?

  • A nice article for you.

  • The blissfully short, quite terribly succient sparknotes is:

    • A "stack protector" is used to prevent exploitation of stack overwrites. the stack protector as implemented in gcc and clang adds an additional guard variable to each function’s stack area.

  • Interesting Drawback To Note:

    "Adding these checks will lead to a little runtime overhead: More stack space is needed, but that is negligible except for really constrained systems...Do you aim for maximum security at the cost of performance? -fstack-protector-all is for you."

• What is sqrtss?

  According to @godbolt:

      Computes the square root of the low single-precision floating-point value
      in the second source operand and stores the single-precision floating-point
      result in the destination operand. The second source operand can be an XMM
      register or a 32-bit memory location. The first source and destination
      operands is an XMM register.
  

• What is a "source operand"?

  • A tutorial can be found here

  • In essence, an operand is a location of data in a computer. Imagine the simple instruction of x+x=y.You need to know what 'x' is, which is the source operand. And where the result will be stored, 'y', which is the destination operand. Notice how the '+' symbol, which is commonly called an 'operation' can be forgotten, because it doesn't matter in this example.

• What is an "XMM register"?

  • An explanation can be found here.

  • It's just a specific type of register. It's primarily used in floating math ( which, surpisingly enough, is the math you are trying to do ).

• What is rsqrtss?

  • Again, according to @godbolt:

    Computes an approximate reciprocal of the square root of the low
    single-precision floating-point value in the source operand (second operand)
    stores the single-precision floating-point result in the destination operand.
    The source operand can be an XMM register or a 32-bit memory location. The
    destination operand is an XMM register. The three high-order doublewords of
    the destination operand remain unchanged. See Figure 10-6 in the Intel® 64 and
    IA-32 Architectures Software Developer’s Manual, Volume 1, for an illustration
    of a scalar single-precision floating-point operation.
    

• What is a "doubleword"?

  • A simple definition.

  • It is a unit of measurement of computer memory, just like 'bit' or 'byte'. However, unlike 'bit' or 'byte', it is not universal and depends on the architectures of the computer.

• What does "Figure 10-6 in the Intel® 64 and IA-32 Architectures Software Developer’s Manual, Volume 1" look like?

  • Here you go.

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Disclaimer: Most of this knowlegde comes from outside sources. I literally install clang just now to help answer your question. I'm not an expert.