Constant folding is related compiler optimizations used by many modern compilers. Constant folding is process of recognizing & evaluating a static expression at compile rather than computing them at runtime .
Questions tagged [constantfolding]
43 questions
75
votes
3 answers
Why 0.1 + 0.2 == 0.3 in D?
assert(0.1 + 0.2 != 0.3); // shall be true
is my favorite check that a language uses native floating point arithmetic.
C++
#include
int main()
{
printf("%d\n", (0.1 + 0.2 != 0.3));
return…
Stas
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57
votes
7 answers
Why an expression instead of a constant, in a C for-loop's conditional?
In many programming competitions I have seen people write this type of for-loop
for(i = 0; i < (1 << 7); i++)
Unless I am missing something, that's the same as
for(i = 0; i < 128; i++)
Why use the (1 << 7) version?
Isn't calculating the condition…
harrythomas
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47
votes
1 answer
Why does GCC implement isnan() more efficiently for C++ than C ?
Here's my code:
int f(double x)
{
return isnan(x);
}
If I #include I get this assembly:
xorl %eax, %eax
ucomisd %xmm0, %xmm0
setp %al
This is reasonably clever: ucomisd sets the parity flag if the comparison of x with itself is…
John Zwinck
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45
votes
2 answers
log(10.0) can compile but log(0.0) cannot with undefined reference?
For the following C source code:
#include
int main(void)
{
double x;
x = log(0.0);
return 0;
}
When I compile with gcc -lm, I got:
/tmp/ccxxANVH.o: In function `main':
a.c:(.text+0xd): undefined reference to…
xuhdev
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13
votes
2 answers
variable length array folded to constant array
const int buf_length = 255;
char buf[ buf_length + 1 ];
snprintf(buf, buf_length, "%d Next on [%s] %s:", channel, station_channel(channel), station_name(channel));
strncat(buf, "(", buf_length - strlen (buf));
strncat(buf, station_country( xmltv…
user1840007
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10
votes
1 answer
C++ constant folding a loop for prime numbers
Having had a look at previous questions 1, 2 , I was wondering if I can force the compiler to perform a constant folding for the following code which prints prime numbers.
#include
using namespace std;
inline bool is_prime(int n)
{
…
ar2015
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10
votes
1 answer
why to avoid constant folding in Java? When?
I saw some codes in slf4j as show below. I don't know why to avoid constant folding in here. Is it necessary to do that? or just best practice. what's the benefit of doing this?
Thanks.
/**
* Declare the version of the SLF4J API this…
easycoder
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9
votes
2 answers
What is constant folding in java compiler?
Possible Duplicate:
is there any concept called “Constant Folding” in java?
Hi
I have come across line Java compiler uses something known as Constant Folding.What is this? and how Does it affect?
giri
- 26,773
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9
votes
4 answers
gcc complex constant folding
It seems that gcc has some limitation on complex constant folding. Here is an example:
static inline unsigned int DJBHash(const char *str)
{
int i;
unsigned int hash = 5381;
for(i = 0; i < strlen(str); i++)
{
hash = ((hash << 5) +…
Amir Gonnen
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8
votes
2 answers
How does O=Deparse work, and does Perl have and fold constant arrays?
I'm wondering, does -MO=Deparse show you all of the Perl optimizations, and why doesn't this get folded in Perl 5.10?
$ perl -MO=Deparse -e'[qw/foo bar baz/]->[0]'
['foo', 'bar', 'baz']->[0];
-e syntax OK
Some on IRC thought that O=Deparse might…
Evan Carroll
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8
votes
2 answers
Subtracting uint and int and constant folding
Based on this interesting question: Addition of int and uint and toying around with constant folding as mentioned in Nicholas Carey's answer, I've stumbled upon a seemingly inconsistent behavior of the compiler:
Consider the following code…
InBetween
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7
votes
1 answer
What are the specific rules for constant folding?
I just realized that CPython seems to treat constant expressions, which represent the same value, differently with respect to constant folding. For example:
>>> import dis
>>> dis.dis('2**66')
1 0 LOAD_CONST 0 (2)
…
a_guest
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6
votes
1 answer
A trick to prevent the compiler from constant folding out an expression
I have a string literal in my program, I'm trying to create an amateur checksum to ensure the string literal was not replaced in the portable executable.
To do this, I create a hash of the string literal, and store it as a integer literal in the…
Thomas
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5
votes
3 answers
Does Java Compiler include String Constant Folding?
I found out that Java supports constant folding of primitive types, but what about Strings?
Example
If I create the following source code
out.write(""
+ ""
+ ""
+ "Easier to read if it is split into multiple…
700 Software
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5
votes
1 answer
Constant folding in the preprocessing stage
I have a piece of C code I need to deobfuscate. It contains a bunch of tricky macros. I ran the code through C preprocessor and indent and now it looks similar to this:
switch (9263 + 1505) {
case 1505 + 41131 + 6729 + 2347:
...
case…
Pavel Zaichenkov
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