What Exactly PHI Instruction Does And How To Use It In LLVM

Trang chủ » Phi Llvm » What Exactly PHI Instruction Does And How To Use It In LLVM

Có thể bạn quan tâm

A phi node is an instruction used to select a value depending on the predecessor of the current block (Look here to see the full hierarchy - it's also used as a value, which is one of the classes which it inherits from).

Phi nodes are necessary due to the structure of the SSA (static single assignment) style of the LLVM code - for example, the following C++ function

void m(bool r, bool y){ bool l = y || r ; }

gets translated into the following IR: (created through clang -c -emit-llvm file.c -o out.bc - and then viewed through llvm-dis)

define void @_Z1mbb(i1 zeroext %r, i1 zeroext %y) nounwind { entry: %r.addr = alloca i8, align 1 %y.addr = alloca i8, align 1 %l = alloca i8, align 1 %frombool = zext i1 %r to i8 store i8 %frombool, i8* %r.addr, align 1 %frombool1 = zext i1 %y to i8 store i8 %frombool1, i8* %y.addr, align 1 %0 = load i8* %y.addr, align 1 %tobool = trunc i8 %0 to i1 br i1 %tobool, label %lor.end, label %lor.rhs lor.rhs: ; preds = %entry %1 = load i8* %r.addr, align 1 %tobool2 = trunc i8 %1 to i1 br label %lor.end lor.end: ; preds = %lor.rhs, %entry %2 = phi i1 [ true, %entry ], [ %tobool2, %lor.rhs ] %frombool3 = zext i1 %2 to i8 store i8 %frombool3, i8* %l, align 1 ret void }

So what happens here? Unlike the C++ code, where the variable bool l could be either 0 or 1, in the LLVM IR it has to be defined once. So we check if %tobool is true, and then jump to lor.end or lor.rhs.

In lor.end we finally have the value of the || operator. If we arrived from the entry block - then it's just true. Otherwise, it is equal to the value of %tobool2 - and that's exactly what we get from the following IR line:

%2 = phi i1 [ true, %entry ], [ %tobool2, %lor.rhs ]

Từ khóa » Phi Llvm