C Coding Techniques for
Intel Architecture Processors
1995, Intel Corporation
This section includes advice for writing software optimized for the whole Intel
Architecture (IA) family of processors, from Intel486 processors, to
PentiumŁł processors, to P6 processors. We classify this type of IA software
as BLENDED.
Intel Architecture Code Optimization
" Use a new technology compiler in application
development
Blended code: A single binary that executes very well on all Intel
Architecture processors
We have seen 25+% performance gain in blended code over the past 2
years
See section on Compiler Information for more details
" Use 32-bit application software where possible
" Aggressive code optimization may force you to re-
optimize for the next version
Blended Intel Architecture code will provide scaleable performance
across processor families i.e. i486"!, Pentium & P6 Processors
(See section on Tuning Trade-offs in this CD for more details.)
Compiler techniques are improving all of the time and it is recommended that
the latest version of a compiler is always used. Even if you don t change your
code at all, a new compiler can improve the performance of your software.
The 386 processor introduced 32-bit registers and newer generations of IA
processors have focussed upon improving this 32-bit software. While 8 and 16
bit software will not run slower on newer CPU generations, it may not speed
up as dramatically as 32-bit software. Use 32-bit memory/operations pointers
wherever possible.
Intel Architecture Optimizations
" PentiumŁł Processor-Specific Optimizations
Branch Prediction (i.e. always select fall through code)
Instruction scheduling (i.e. instruction pairing)
Use FXCG to optimize floating point performance
" For the P6:
Use PentiumŁł Processor branch prediction algorithm as a baseline, with
better prediction algorithms imminent
Remove Self Modifying Code
Remove Partial Stalls
" Next generation processors will implement register renaming
" Register renaming predicates a performance issue with intermixed 8,
16 and 32 bit registers (i.e. writing AL followed by reading EAX is a
stall)
Align Data References
" Ensure data alignment rules are followed
Pentium processor instruction paving doesn t hurt, but it s not necessary
There are some general optimizations that can improve the execution speed of
processors with branch prediction -- important for the PentiumŁł processor and
vital for the P6. Try to make the general flow of the software as a straight path
-- divert the flow for exception conditions or other rarely executed code.
Subroutines, or other functions, should have a return statement, not a JMP
instruction. This will make them more predictable.
Some instruction pairing can improve the PentiumŁł processors performance.
While this doesn t particularly help the P6 (instruction re-scheduling is done in
hardware), it doesn t hurt P6 performance either. The Intel 486 processor
also is not negatively impacted by Pentium processor pairing.
Short To Integer
" Short variables shouldn t be loop index variables in a 32 bit
program
" Data Size Override Prefixes Will be Generated
" Prefixes Limit Pairing and Take Longer to Execute
In a 32-bit applications, using a short as a part of a loop will cause size
override prefixes to be incorporated. These overrides take longer to fetch,
decode and execute.
ALWAYS use 32 bit integers as loop variables -- your software will run faster.
Short To Integer
" Original Code " Improved Code
void this_routine( void this_routine(
float *a, float **b, int n) float *a, float **b, int n)
{ {
short i; int i;
short k = n; int k = n;
for (i=0; i
a[i] += b[0][i]; a[i] += b[0][i];
} }
} }
Example for previous page.
Temp Variables To Clarify
Pointer Dependences
" Compiler optimizations can be limited by potential
dependence conflicts with pointers
" Instruction reordering, or scheduling, for better
pairing and address generation is often affected
Multiple lilnes of C code that have the same variables used as pointer
references cause the code/data dependency tree to be extended, reduce
parallelism and lower performance.
Reorder instructions and introduce temporary pointers where possible.
Temps To Clarify Pointers
" Original Code " Improved Code
void this_routine( void this_routine(
float *a, float **b, int n) float *a, float **b, int n)
{ {
*a++ += b[0][n]; register float temp_1;
*a += b[0][n-1]; register float temp_2;
} temp_1 = *a + b[0][n];
temp_2 = *(a+1) + b[0][n-1];
(a == &b[0][n-1] ??)
*a++ = temp_1;
*a = temp_2;
}
Note: Calculations of temp1 & temp2 may be interleaved.
Example for previous page.
Loop Invariant Motion
" Loop Invariant Pointer Dereferences can generate
unnecessary code
" Pointer dereference may be done outside the loop
to a temp variable
Loop invariant pointer dereferences that are using/accessing normal stack
based variables may generate inefficient/slow code. The movement of this
code to temporary variables and possibly a register based implementation will
improve performance.
Loop Invariant Motion
" Original Code " Optimized Code
void test_post (int n, int *a, int b) void test_post (int n, int *a, int b)
{ {
int lim; int lim;
register int temp_a;
lim = n;
while (lim--) lim = n;
{ temp_a = *a;
*a += b; while (lim--)
} {
} temp_a += b;
}
*a = temp_a;
}
Example for previous page.
Loop Unrolling
" Loop Unrolling
Can save in loop overhead
Provides the compiler more opportunity to optimize by
interleaving instructions
" Unroll the loop by doing the following:
1. Replicate the body of the loop.
2. Adjust the index expression if needed.
3. Adjust the loop iteration s control statements.
Loop unrolling is a standard compiler technique that provides the opportunity
for higher performance by providing the compiler with a larger basic block to
optimize and thus more opportunity. This unrolling also allows turning based
on cache/memory architecture to be more controllable. Basic rule: the smaller
the size of the loop, the higher the priority for unrolling. Consideration: loop
unrolling will likely help the PentiumŁł processor more than the P6.
Loop Unrolling
" Original Code
" Optimized Code
void test_it(
void test_it(
int *a, int* c, int n)
int *a, int* c, int n)
{
{
int i;
int i;
for (i=0; i<99; i++)
for (i=0; i<99; i+=3)
{
{
a[i] = c[i] ;
a[i] = c[i];
}
a[i+1] = c[i+1];
}
a[i+2] = c[i+2];
}
}
Example for previous page. (While this does not hurt P6 execution time, it is
not always necessary since the P6 does many aspects of this operation
automatically via its Dynamic Execution core.)
Loop Invariant If Statements
" Moving If statements out of loops can save
execution time
" Replicate the loop to produce desired effect
The movement of loop invariants outside the loop core will reduce the
unnecessarily repetitive execution of those instructions. This may result in
loop repetition or duplication and the resultant larger code size, but execution
performance will improve.
Loop Invariant Ifs
" Original Code " Optimized Code
void test_if( void test_if(
int *a, int *p, int *q, int n) int *a, int *p, int *q, int n)
{ int i; { int i;
for(i=0; i{ for(i=0; iif (putp==1) {
a[i]=p[i]+q[i]; a[i]=p[i]+q[i];
else }
a[i]=p[i]-q[i]; else
} for(i=0; i} {
a[i]=p[i]-q[i];
}
}
Example for previous page.
Loop Initialization
" Use a well-tuned library routine like memset to
initialize arrays.
" May Improve performance of the application significantly.
Libraries are a very good place for optimization, allowing tuning to be
implemented without a complete recompile of the application. Relinking is
only necessary for regeneration. The libraries provide a potential isolation of
the application from processor/architecture-specific requirements.
Libraries should be scanned for optimal routines that may be incorporated into
the normal function required by an application (i.e., memset for array
initialization).
Loop Initialization
" Original Code " Optimized Code
void test_it( void test_it(
char *a, char c, int n) char *a, char c, int n)
{ {
int i; memset(a, c, n);
for (i=0; i{
a[i] = c;
}
}
Memset is much faster mechanism for replicating a value through memory.
Loop Invariant Division
" Division is Much Slower than Multiplication
" Calculate Reciprocal outside of loop and use
Multiply inside
Another good tip to speed execution: If possible, move any loop-based
division to a multiply by reciprocal implementation.
Loop Invariant Division
" Original Code " Optimized Code
void test_it( void test_it(
float *a, float* c, int n) float *a, float* c, int n)
{ {
int i; int i; float denom;
float denom = *c; denom = 1.0 / (*c);
for (i=0; i{ {
a[i] = a[i] / denom; a[i] = a[i] * denom;
} }
} }
Example for previous page.
Logical OR Conversion
" Testing for equality with small
integers using OR (||)
" Table lookup can avoid several
branches
Another useful suggestion.
Logical OR Conversion
" Optimized Code
" Original Code
void sub(int *, int*);
void sub(int *, int*);
int test_table[16]={0,1,0,0,1,0,0,1,
void test_it(int * a, int *b, int signif)
0,0,1,0,0,1,0,0};
{
void test_it(
if (signif == 1 || signif == 4 ||
int * a, int *b, int signif)
signif == 7 || signif == 10 ||
{
signif == 13)
if (test_table[signif])
{
sub(a,b);
sub(a,b);
else
}else
sub(b,a);
sub(b,a);
}
}
Example for previous page.
Call to Error
" Infrequently executed code can take up instruction
cache space and bus bandwidth needlessly
" Moving infrequently used code out of line can improve
performance
A final suggestion: All IA processors perform best when they are allowed to
prefetch decode or execute in a straight line with no branches to break the
pipeline.
The movement of infrequently executed code (e.g., exception/error handlilng
code) will allow the maximum prefetch/decode/execute bandwidth to be
exposed.
Call to Error
" Original Code " Optimized Code
void test_it( char *mem, int flag) void test_it( char *mem, int flag)
{ {
if (flag < 0) error ("flag is negative"); if (flag < 0) goto flag_err;
dummy (flag, &status); dummy (flag, &status);
if (status != OK) if (status != Ok)
error ( dummy failed. ); goto dummy_err;
return; return;
} flag_err:
error ("flag is negative"); return;
dummy_err:
error ( dummy failed. ); return;}
Example for previous page.
For more information, see the 32-bit Optimization Guide in this CD.
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