Hello
I compiled the following program using the web interface
#include <stdio.h>
#include <stdlib.h>
int main(int argc, char **argv) {
int a; int b; int c; int d;
int X = 10;
a = 777;
b = a | (atoi(argv[1]));
c = b | (atoi(argv[2]));
a = c | (atoi(argv[4]));
b = a | (atoi(argv[5]));
d = b | (atoi(argv[6]));
a = d | (atoi(argv[7]));
b = a | (atoi(argv[8]));
c = b | (atoi(argv[9]));
if (c) {
X = 2000;
}
printf("%d\n", X);
return 0;
}
It is easy to see that the condition of the if statement will be satisfied,
but non of the optimization levels of the web interface seem to understand
this. If I remove all OR operations except the first two, then the code will
be optimized as expected. I guess this is because InstCombine, SCCP or
whatever pass who is responsible for this optimization is called a constant
number of times rather than "enough times". (For example call it in a
loop
until it reports no simplification was done). Now my questions:
1- Is my understanding correct that this optimization is not done? Maybe if
I choose flags correctly it is done?
2- Do you think I can use LLVM library in way that this kind of optimization
is performed? For example is it possible that I call passes responsible for
this in a loop and then ask them in each iteration if they did something
useful and then I stop when they say they didn't find something new?
Best
Ehsan
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On Thu, Jun 17, 2010 at 4:30 PM, Ehsan Amiri <ehsanamiri at gmail.com> wrote:> Hello > > I compiled the following program using the web interface > > #include <stdio.h> > #include <stdlib.h> > > int main(int argc, char **argv) { > int a; int b; int c; int d; > int X = 10; > a = 777; > b = a | (atoi(argv[1])); > c = b | (atoi(argv[2])); > a = c | (atoi(argv[4])); > b = a | (atoi(argv[5])); > d = b | (atoi(argv[6])); > a = d | (atoi(argv[7])); > b = a | (atoi(argv[8])); > c = b | (atoi(argv[9])); > if (c) { > X = 2000; > } > printf("%d\n", X); > return 0; > } > > > It is easy to see that the condition of the if statement will be satisfied, > but non of the optimization levels of the web interface seem to understand > this. If I remove all OR operations except the first two, then the code will > be optimized as expected. I guess this is because InstCombine, SCCP or > whatever pass who is responsible for this optimization is called a constant > number of times rather than "enough times". (For example call it in a loop > until it reports no simplification was done). Now my questions: > > 1- Is my understanding correct that this optimization is not done? Maybe if > I choose flags correctly it is done?Yes; instcombine normally handles situations like this, but there's a recursion limit so pathological cases don't blow the stack and/or cause very long running times. In this particular case, I believe the limit is in llvm::ComputeMaskedBits.> 2- Do you think I can use LLVM library in way that this kind of optimization > is performed? For example is it possible that I call passes responsible for > this in a loop and then ask them in each iteration if they did something > useful and then I stop when they say they didn't find something new?I don't think there's any way to make a pass perform this optimization without modifying the LLVM code or writing your own pass to perform it. Is this really an important case for you? -Eli
> recursion limit so pathological cases don't blow the stack and/or > cause very long running times. In this particular case, I believe the > limit is in llvm::ComputeMaskedBits. > > I did a few other tests, it does optimization upto six ORs and stopsoptimizing when there are seven or more OR instructions.> > I don't think there's any way to make a pass perform this optimization > without modifying the LLVM code or writing your own pass to perform > it. Is this really an important case for you? > > Optimizing Boolean expressions are really important for me. I may have longstraight lines of Boolean operations and then I may want to optimize based on setting a variable to TRUE or FALSE. I think depths larger than six may happen sometimes in my application. Something extra that I may need is this. I have a line of code a = b | c. Now I know for some reason that a = FALSE. This implies that b = FALSE and c = FALSE as well. Now I want to propagate all three values in the code and simplify as much as possible. Does by any chance LLVM do this? Best Ehsan -------------- next part -------------- An HTML attachment was scrubbed... URL: <http://lists.llvm.org/pipermail/llvm-dev/attachments/20100617/7d0f71f8/attachment.html>