https://llvm.org/bugs/show_bug.cgi?id=31929
Bug ID: 31929
Summary: branch condition should be known from llvm.assume
Product: libraries
Version: trunk
Hardware: PC
OS: All
Status: NEW
Severity: normal
Priority: P
Component: Scalar Optimizations
Assignee: unassignedb...@nondot.org
Reporter: spatel+l...@rotateright.com
CC: llvm-bugs@lists.llvm.org
Classification: Unclassified
Filing this based on the request in:
https://reviews.llvm.org/D28204
These are purposely minimized to be instcombine tests. If we decide that the
functionality doesn't belong there, then the tests may require additional
instructions to thwart other transformations (phi -> select) and/or trigger
from a different pass (LICM?).
-----------------------------------------------------------------------------
declare void @llvm.assume(i1)
define i8 @assume_guarantees_notnull(i8* %x) {
entry:
%notnull = icmp ne i8* %x, null
tail call void @llvm.assume(i1 %notnull)
br i1 %notnull, label %if, label %endif <--- condition is known true
if:
%ld = load i8, i8* %x
br label %endif
endif:
%sel = phi i8 [ 0, %entry ], [ %ld, %if ]
ret i8 %sel
}
Depending on how we want to solve this, it may be relevant to use a non-pointer
value in the icmp:
define i8 @assume_guarantees_notnull(i8 %y, i8* %x) {
entry:
%notnull = icmp ne i8 %y, 0 <--- type shouldn't matter?
tail call void @llvm.assume(i1 %notnull)
br i1 %notnull, label %if, label %endif
if:
%ld = load i8, i8* %x, align 1
br label %endif
endif:
%sel = phi i8 [ 0, %entry ], [ %ld, %if ]
ret i8 %sel
}
----------------------------------------------------------------------------
For reference, we handle the 'select' variant of these examples in instcombine
by calling computeKnownBits when we know there might be an assumption that
defines the condition. That was bug 31512 / https://reviews.llvm.org/rL291915 .
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