LVI does not resolve condition value in destination blocks of conditional branches

[llvmbot]

Bugzilla Link	9004
Resolution	FIXED
Resolved on	Nov 21, 2017 04:25
Version	trunk
OS	Linux
Reporter	LLVM Bugzilla Contributor
CC	@fhahn,@nunoplopes

Extended Description

Run "opt -gvn" on the following testcase. It gets that "ret i1 %cond2" returns
false, but it doesn't see that the a priori easier "ret i1 %cond" returns true!

define i1 @​foo(i32 %x) {
%cond = icmp eq i32 %x, 0
br i1 %cond, label %iftrue, label %iffalse
iftrue:
ret i1 %cond
iffalse:
%cond2 = icmp eq i32 %x, 0
ret i1 %cond2
}

[llvmbot]

Interestingly -correlated-propagation has exactly the same problem: it catches
the harder case but not the easier one!

[llvmbot]

By the way, this phenomenon is the cause of a regression due to Early-CSE that
I noticed. I have some IR with the "hard" case. Early-CSE turns this into the
"easy" case - and then the simplification is missed. In my case the value isn't
being returned, it's the condition for a condition branch so this leads to the
branch not being eliminated any more.

[llvmbot]

Actually, it looks like SCCP is probably the place that should be doing this
(but isn't).

[llvmbot]

In case it wasn't clear, the general problem seems to be that if you have a
conditional branch, no-one seems to be replacing the branch condition with the
appropriate constant in the destination blocks.

[llvmbot]

According to Chris this is a job for LVI, though I'm not totally convinced.

[llvmbot]

GVN gets this now.

[fhahn]

correlated value propagation gets this using LVI. I am closing this now, please reopen in case I missed anything.

opt -correlated-propagation lvi2.ll -S
; ModuleID = 'lvi2.ll'
source_filename = "lvi2.ll"

define i1 @​foo(i32 %x) {
%cond = icmp eq i32 %x, 0
br i1 %cond, label %iftrue, label %iffalse

iftrue: ; preds = %0
ret i1 true

iffalse: ; preds = %0
ret i1 false
}