undef through phi doesn't collapse to undef when it ought

[nlewycky]

Bugzilla Link	717
Resolution	LATER
Resolved on	Mar 01, 2008 20:52
Version	trunk
OS	All

Extended Description

Expressions involving undef get collapsed into undef properly, except for PHI
nodes. The following code should compile into "ret int undef". Instead, LLVM
produces "ret int 0":

int f()
{
int x = 4;
int y;
if (x == 3) y = 0;
return y;
}

[lattner]

Verified. This is a missed optimization.

[lattner]

I've looked into this, and it doesn't seem possible to fix in a straight-forward way, and doesn't seem
very critical. The problem is that we have this code going into the -mem2reg pass:

int %f() {
entry:
%result = alloca int ; <int*> [#uses=2]
%x = alloca int ; <int*> [#uses=2]
%y = alloca int ; <int*> [#uses=2]
store int 4, int* %x
%tmp.0 = load int* %x ; [#uses=1]
%tmp.1 = seteq int %tmp.0, 3 ; [#uses=2]
%tmp.2 = cast bool %tmp.1 to int ; [#uses=0]
br bool %tmp.1, label %then, label %endif
then: ; preds = %entry
store int 0, int* %y
br label %endif
endif: ; preds = %then, %entry
%tmp.3 = load int* %y ; [#uses=1]
store int %tmp.3, int* %result
%tmp.4 = load int* %result ; [#uses=1]
ret int %tmp.4
}

Because we haven't built SSA form yet, the dead code elimination stuff doesn't see the block is dead
before this.

When building SSA in mem2reg, it notices it's about to create a phi(undef,0), and simplifies it to 0. This
is an important pruning heuristic, but causes it to miss the optimization. I don't see how this
enhancement can be done without disabling the heuristic, which would have a negative impact on
normal code at the expense of this minor optimization.

-Chris

[nlewycky]

Why not allow mem2reg to produce the phi(undef,x) then add another pass after
the DCE which collapses phi(undef,x) to undef?

It might make other passes in between mem2reg and DCE slower as the tree would
be larger than before, but there would be no negative impact on resulting code,
would there?

[lattner]

I moved this minor optzn into the README.txt file:
http://lists.cs.uiuc.edu/pipermail/llvm-commits/Week-of-Mon-20080225/059042.html