Question

For a typical modern compiler on modern hardware, will the ? : operator result in a branch that affects the instruction pipeline?

In other words which is faster, calling both cases to avoid a possible branch:

bool testVar = someValue(); // Used later.
purge(white);
purge(black);

or picking the one actually needed to be purged and only doing it with an operator ?::

bool testVar = someValue();
purge(testVar ? white : black);

I realize you have no idea how long purge() will take, but I’m just asking a general question here about whether I would ever want to call purge() twice to avoid a possible branch in the code.

I realize this is a very tiny optimization and may make no real difference, but would still like to know. I expect the ?: does not result in branching, but want to make sure my understanding is correct.

Answer 1

The CMOV (Conditional MOVe) instruction has been part of the x86 instruction set since the Pentium Pro. It is rarely automatically generated by GCC because of compiler options commonly used and restrictions placed by the C language. A SETCC/CMOV sequence can be inserted by inline assembly in your C program. This should only be done is cases where the conditional variable is a randomly oscillating value in the inner loop (millions of executions) of a program. In non-oscillating cases and in cases of simple patterns of oscillation, modern processors can predict branches with a very high degree of accuracy. In 2007, Linus Torvalds suggested here to avoid use of CMOV in most situations.

Intel describes the conditional move in the Intel(R) Architecture Software Developer’s Manual, Volume 2: Instruction Set Reference Manual:

The CMOVcc instructions check the state of one or more of the status
flags in the EFLAGS register (CF, OF, PF, SF, and ZF) and perform a
move operation if the flags are in a specified state (or condition). A
condition code (cc) is associated with each instruction to indicate
the condition being tested for. If the condition is not satisfied, a
move is not performed and execution continues with the instruction
following the CMOVcc instruction.

These instructions can move a 16- or 32-bit value from memory to a
general-purpose register or from one general-purpose register to
another. Conditional moves of 8-bit register operands are not
supported.

The conditions for each CMOVcc mnemonic is given in the description
column of the above table. The terms “less” and “greater” are used for
comparisons of signed integers and the terms “above” and “below” are
used for unsigned integers.

Because a particular state of the status flags can sometimes be
interpreted in two ways, two mnemonics are defined for some opcodes.
For example, the CMOVA (conditional move if above) instruction and the
CMOVNBE (conditional move if not below or equal) instruction are
alternate mnemonics for the opcode 0F 47H.