ARM Compiler toolchain v4.1 for µVision Using the Compiler

Compiler Coding Practices
5.17 Comparison of pure and impure functions
The use of the __pure keyword is illustrated in the two sample routines of Table 5-7. Both
routines call a function fact() to calculate the sum of n! and n!. fact() depends only on its input
argument n to compute n!. Therefore, fact() is a pure function.
The first routine shows a naive implementation of the function fact(), where fact() is not
declared __pure. In the second implementation, fact() is qualified as __pure to indicate to the
compiler that it is a pure function.
Table 5-7 C code for pure and impure functions
A pure function not declared __pure
int fact(int n)
{
int f = 1;
while (n > 0)
f *= n--;
return f;
}
int foo(int n)
{
return fact(n)+fact(n);
}
A pure function declared __pure
int fact(int n) __pure
{
int f = 1;
while (n > 0)
f *= n--;
return f;
}
int foo(int n)
{
return fact(n)+fact(n);
}
Table 5-8 shows the corresponding disassembly of the machine code produced by the compiler
for each of the sample implementations of Table 5-7, where the C code for each implementation
has been compiled using the option -O2, and inlining has been suppressed.
Table 5-8 Disassembly for pure and impure functions
A pure function not declared __pure
A pure function declared __pure
fact PROC
...
foo PROC
MOV
PUSH
BL
MOV
MOV
BL
ADD
POP
ENDP
r3, r0
{lr}
fact
r2, r0
r0, r3
fact
r0, r0, r2
{pc}
fact PROC
...
foo PROC
PUSH
BL
LSL
POP
ENDP
{lr}
fact
r0,r0,#1
{pc}
In the disassembly of function foo() in Table 5-8 where fact() is not qualified as __pure, fact()
is called twice because the compiler does not know that the function is a candidate for Common
Subexpression Elimination (CSE). In contrast, in the disassembly of foo() in Table 5-8 where
fact() is qualified as __pure, fact() is called only once, instead of twice, because the compiler
has been able to perform CSE when adding fact(n) + fact(n).
5.17.1 See also
Concepts
• Functions that return the same result when called with the same arguments on page 5-24.
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