Description
Rust in which we spotted this originally: https://rust.godbolt.org/z/Knr5jdEY5, cc rust-lang/rust#139098 (comment)
Take this input IR:
define noundef range(i8 0, 14) i8 @match0(i8 noundef range(i8 0, 3) %0) unnamed_addr {
start:
%1 = icmp eq i8 %0, 2
%_2 = select i1 %1, i64 1, i64 0
%2 = trunc nuw i64 %_2 to i1
br i1 %2, label %bb2, label %bb3
bb2: ; preds = %start
br label %bb4
bb3: ; preds = %start
%b = trunc nuw i8 %0 to i1
%3 = zext i1 %b to i8
br label %bb4
bb4: ; preds = %bb2, %bb3
%_0.sroa.0.0 = phi i8 [ 13, %bb2 ], [ %3, %bb3 ]
ret i8 %_0.sroa.0.0
}Today https://llvm.godbolt.org/z/vnfGGxfbr, --passes=instcombine transforms that to
source_filename = "/app/example.ll"
define noundef range(i8 0, 14) i8 @match0(i8 noundef range(i8 0, 3) %0) unnamed_addr {
%1 = icmp eq i8 %0, 2
br i1 %1, label %bb2, label %bb3
bb2: ; preds = %start
br label %bb4
bb3: ; preds = %start
%2 = and i8 %0, 1 ; 😢
br label %bb4
bb4: ; preds = %bb3, %bb2
%_0.sroa.0.0 = phi i8 [ 13, %bb2 ], [ %2, %bb3 ]
ret i8 %_0.sroa.0.0
}However it should not be adding that and i8 %0, 1 -- the trunc nuw means it ought to know it doesn't need to do that.
It should instead be emitting https://alive2.llvm.org/ce/z/oN3dig
define noundef range(i8 0, 14) i8 @tgt(i8 noundef range(i8 0, 3) %0) unnamed_addr {
start:
%1 = icmp eq i8 %0, 2
br i1 %1, label %bb2, label %bb3
bb2: ; preds = %start
br label %bb4
bb3: ; preds = %start
br label %bb4
bb4: ; preds = %bb3, %bb2
%_0.sroa.0.0 = phi i8 [ 13, %bb2 ], [ %0, %bb3 ]
ret i8 %_0.sroa.0.0
}Note that the correctness for this doesn't depend on the range information; removing that from the parameter it still validates https://alive2.llvm.org/ce/z/_ed2Ch.
(If you run a full -O2 or -O3 then CVP will get rid of the and, but that's not run in -O1. And InstCombine has the information it needs to do it correctly anyway, AFAICT, so it just should.)