[LLVM] bug: branch with a single target and different arguments

[wangweixuan]

I believe Koopa IR allows br instructions where both branches target the same basic block. I am interested when they carry different arguments:

fun @foo() {
%entry:
  br 0, %end(1), %end(2)

%end(%r: i32):
  ret %r       // returns 2
}

libkoopa cannot faithfully lower it to LLVM IR:

define i32 @foo() {
$entry:
  %$0 = icmp ne i32 0, 0
  br i1 %$0, label %$end, label %$end

$end:
  %$r = phi i32 [1, %$entry]
  ret i32 %$r       ; won't compile using `llc', returns 1 using `clang'
}

[MaxXSoft]

I believe Koopa IR allows br instructions where both branches target the same basic block.

True.

I am interested when they carry different arguments:

I actually overlooked this issue before, my fault.

This can be handled in two ways:

1. When there is a branch instruction with two identical target basic blocks and two sets of BB parameters, it performs a selection first, passes selected values as BB parameters, and then transfers the control flow to the target block:

br %cond, %target(%a0, %a1, ...), %target(%b0, %b1, ...)

is actually:

%p0 = select %cond, %a0, %b0
%p1 = select %cond, %a1, %b1
...
jump %target(%p0, %p1, ...)

2. This kind of usage is illegal.

From the example you provided, it seems that your understanding is based on the first approach. However, since the basic block parameter is an alternative way of representing Phi functions, I would prefer to disable this usage for the sake of IR semantic orthogonality.

Thanks for your feedback. I'll add some additional checks in koopa/libkoopa later.