https://bugs.llvm.org/show_bug.cgi?id=39518
Bug ID: 39518
Summary: [InstCombine] optimize compares of abs/nabs into
union/intersection of compares
Product: libraries
Version: trunk
Hardware: PC
OS: All
Status: NEW
Severity: enhancement
Priority: P
Component: Scalar Optimizations
Assignee: unassignedb...@nondot.org
Reporter: spatel+l...@rotateright.com
CC: llvm-bugs@lists.llvm.org
Adjusting the bug-inducing code from bug 39510 to the more likely (?) pattern
shows an opportunity:
define i1 @abs_test(i32 %a) {
%cmp = icmp slt i32 %a, 0
%sub = sub nsw i32 0, %a
%abs = select i1 %cmp, i32 %sub, i32 %a
%r = icmp eq i32 %abs, 2
ret i1 %r
}
This can be reduced to logic-of-icmps:
https://rise4fun.com/Alive/FjLJ
define i1 @abs_test_better(i32 %a) {
%cmp1 = icmp eq i32 %a, 2
%cmp2 = icmp eq i32 %a, -2
%r = or i1 %cmp1, %cmp2
ret i1 %r
}
----------------------------------------------------------------------------
More generally, we should be forming union/intersection patterns:
https://rise4fun.com/Alive/Rqq0
Name: union
%cmp = icmp slt i32 %a, 0
%sub = sub nsw i32 0, %a
%abs = select i1 %cmp, i32 %sub, i32 %a
%r = icmp sgt i32 %abs, 42
=>
%cmp1 = icmp sgt i32 %a, 42
%cmp2 = icmp slt i32 %a, -42
%r = or i1 %cmp1, %cmp2
Name: intersection
%cmp = icmp slt i32 %a, 0
%sub = sub nsw i32 0, %a
%abs = select i1 %cmp, i32 %sub, i32 %a
%r = icmp slt i32 %abs, 42
=>
%cmp1 = icmp slt i32 %a, 42
%cmp2 = icmp sgt i32 %a, -42
%r = and i1 %cmp1, %cmp2
----------------------------------------------------------------------------
I don't have an immediate plan to work on this, but it probably needs a codegen
reversal before the IR canonicalization based on whether a target has a legal
ISD::ABS node.
--
You are receiving this mail because:
You are on the CC list for the bug._______________________________________________
llvm-bugs mailing list
llvm-bugs@lists.llvm.org
http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-bugs
