[ipsccp] Sparse conditional IP constant propagation

[lattner]

Bugzilla Link	415
Resolution	FIXED
Resolved on	Feb 22, 2010 12:47
Version	1.0
OS	All

Extended Description

Brian pointed out that the Olden/mst benchmark has code that basically looks
like this:

main() {
Do_all_BlueRule(..., 1, 0);
}

void Do_all_BlueRule(..., int nproc, int pn) {
if (nproc > 1) {
Do_all_BlueRule(nproc/2, nproc/2+pn);
... A BUNCH OF OTHER STUFF ...
}

bar();
}

The observation is that Do_all_BlueRule contains a bunch of code that is
dynamically dead, but it requires a smarter IPCP algorithm to notice it (the
unreachable recursive call causes us to miss this).

It seems like an aggressive IPCP coupled with SCCP'y techniques could solve this
pretty easily.

-Chris

[lattner]

assigned to @lattner

[lattner]

There are some notes for this bug here:
http://nondot.org/sabre/LLVMNotes/IP-SCCP.txt

-Chris

[lattner]

Here's a trivial testcase this should handle:

implementation ; Functions:

internal int %foo(int %X) {
%Y = call int %foo( int %X ) ; [#uses=1]
%Z = add int %Y, 1 ; [#uses=1]
ret int %Z
}

void %bar() {
call int %foo( int 17 ) ; :0 [#uses=0]
ret void
}

IPCP should realize that %X is always '17'. The current, simple IPCP, should
detect and handle this too. I wish it already did, it would help pool
allocation. :(

-Chris

[lattner]

ok, another note. Simple IPCP now handles the simple case above.

[lattner]

Initial implementation:
http://mail.cs.uiuc.edu/pipermail/llvm-commits/Week-of-Mon-20041206/022044.html

There is more to come for this PR, stay tuned!

-Chris

[lattner]

This is now implemented, named -ipsccp.

-Chris