Will bit-shift by zero bits work correctly?

27,347

Solution 1

It is certain that at least one C++ compiler will recognize the situation (when the 0 is known at compile time) and make it a no-op:

Source

inline int shift( int what, int bitcount)
{
  return what >> bitcount ;
}

int f() {
  return shift(42,0);
}

Compiler switches

icpc -S -O3 -mssse3 -fp-model fast=2 bitsh.C

Intel C++ 11.0 assembly

# -- Begin  _Z1fv
# mark_begin;
       .align    16,0x90
        .globl _Z1fv
_Z1fv:
..B1.1:                         # Preds ..B1.0
        movl      $42, %eax                                     #7.10
        ret                                                     #7.10
        .align    16,0x90
                                # LOE
# mark_end;
        .type   _Z1fv,@function
        .size   _Z1fv,.-_Z1fv
        .data
# -- End  _Z1fv
        .data
        .section .note.GNU-stack, ""
# End

As you can see at ..B1.1, Intel compiles "return shift(42,0)" to "return 42."

Intel 11 also culls the shift for these two variations:

int g() {
  int a = 5;
  int b = 5;
  return shift(42,a-b);
}

int h(int k) {
  return shift(42,k*0);
}

For the case when the shift value is unknowable at compile time ...

int egad(int m, int n) {
  return shift(42,m-n);
}

... the shift cannot be avoided ...

# -- Begin  _Z4egadii
# mark_begin;
       .align    16,0x90
        .globl _Z4egadii
_Z4egadii:
# parameter 1: 4 + %esp
# parameter 2: 8 + %esp
..B1.1:                         # Preds ..B1.0
        movl      4(%esp), %ecx                                 #20.5
        subl      8(%esp), %ecx                                 #21.21
        movl      $42, %eax                                     #21.10
        shrl      %cl, %eax                                     #21.10
        ret                                                     #21.10
        .align    16,0x90
                                # LOE
# mark_end;

... but at least it's inlined so there's no call overhead.

Bonus assembly: volatile is expensive. The source ...

int g() {
  int a = 5;
  volatile int b = 5;
  return shift(42,a-b);
}

... instead of a no-op, compiles to ...

..B3.1:                         # Preds ..B3.0
        pushl     %esi                                          #10.9
        movl      $5, (%esp)                                    #12.18
        movl      (%esp), %ecx                                  #13.21
        negl      %ecx                                          #13.21
        addl      $5, %ecx                                      #13.21
        movl      $42, %eax                                     #13.10
        shrl      %cl, %eax                                     #13.10
        popl      %ecx                                          #13.10
        ret                                                     #13.10
        .align    16,0x90
                                # LOE
# mark_end;

... so if you're working on a machine where values you push on the stack might not be the same when you pop them, well, this missed optimization is likely the least of your troubles.

Solution 2

According to K&R "The result is undefined if the right operand is negative, or greater than or equal to the number of bits in the left expression's type." (A.7.8) Therefore >> 0 is the identity right shift and perfectly legal.

Solution 3

It will work correctly on any widely used architecture (I can vouch for x86, PPC, ARM). The compiler will not be able to reduce it to a noop unless the function is inlined.

Solution 4

About the correctness of arg << 0 or arg >> 0, no problem, absolutely fine.

About the eventual optimizations: This will not be reduced to a >nop< when called with a constant what=0 and/or bitcount=0, unless you declare it as inline and choose optimizations (and your compiler of choice understands what inline is).

So, bottom line, optimize this code by conditionally calling the function only if the OR of arguments is non zero (about the fastest way I figure to test that both args are non-zero).

Solution 5

To make the function somewhat self documenting, you may want to change bitCount to unsigned to signify to callers that a negative value is not valid.

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27,347
sharptooth
Author by

sharptooth

Updated on July 23, 2020

Comments

  • sharptooth
    sharptooth over 3 years

    Say I have a function like this:

    inline int shift( int what, int bitCount )
    {
        return what >> bitCount;
    }
    

    It will be called from different sites each time bitCount will be non-negative and within the number of bits in int. I'm particularly concerned about call with bitCount equal to zero - will it work correctly then?

    Also is there a chance that a compiler seeing the whole code of the function when compiling its call site will reduce calls with bitCount equal to zero to a no-op?

  • paxdiablo
    paxdiablo over 14 years
    Of course, having said that, I've never seen a compiler in the wild do that level of optimization - I suspect the return on investment wouldn't be enough to warrant it.
  • paxdiablo
    paxdiablo over 14 years
    @jpinto3912, I think question is asking what happens when the shift is zero, not the value (as your answer seems to assume).
  • Admin
    Admin over 14 years
    He is not shifting zero, he is shifting something zero times.
  • peterchen
    peterchen over 14 years
    Pax, VC9 does "out of the box" (default release settings) for: ---- int Adjust(int x, int shift) { if (shift > 0) { /* large block that prevents inlining */ } return x << shift; } ---- Even when calling from a different translation unit, when passing shift=0, the call and the >> will be culled. Quite impressive!
  • Crashworks
    Crashworks over 14 years
    I should have been clearer and said "inlined, explicity or implicitly." What you're seeing there is the compiler pushing something to inline even though it's not been marked. If the function isn't inline then by definition it's called by stack convention and can't be elided (unless inlined later in LTCG).
  • Stobor
    Stobor about 14 years
    @Pax, @Neil: if you're shifting zero, you'll also effectively get a no-op. The question asks about one case, the answer notes both cases.
  • LaSul
    LaSul about 14 years
    Surely the shift by zero would be faster than a potential branch with an OR?
  • Lumen
    Lumen almost 14 years
    @kibibu, likely, but that's not the scenario. It's pushing args, calling a function, shifting by zero, and pop stack for return. If the situation is often a zero-shift, that is a lot slower than OR and IFNZ prior to calling. Anyway, this is just in case your compiler don't know what's "inline" (or you have a tiny code space req).
  • Konrad Borowski
    Konrad Borowski over 9 years
    Modern compilers don't even need inline to inline, when the function is really simple, and it's defined in the same compilation unit.
  • Johannes Overmann
    Johannes Overmann over 4 years
    This does not answer whether shifting by zero is defined behavior or not.