Fix multi-edge phi node code generation

It is possible, after optimziations for the two branches of a conditional
to go to the same basic block. We have an IR invariant that requires the
incoming value to be the same for both incoming edges in this case. LLVM
has the same invariant. However, translating from a julia-incoming-value
to an LLVM one may require additional code generation, and without further
GVN, LLVM does not know that the two values are identical and thus believes
its invariants are violated. The fix is relatively straightforward. Since
we don't have user-inserted switch terminators, we can simply emit a
conditional branch with equal successors as unconditional branches and
skip codegening one of the phi paths. This avoids the issue and also
generates less code in this case.

Fixes #27594

src/codegen.cpp

@@ -6242,7 +6242,10 @@ static std::unique_ptr<Module> emit_function(
             workstack.push_back(lname - 1);
             BasicBlock *ifnot = BB[lname];
             BasicBlock *ifso = BB[cursor+2];
-            ctx.builder.CreateCondBr(isfalse, ifnot, ifso);
+            if (ifnot == ifso)
+                ctx.builder.CreateBr(ifnot);
+            else
+                ctx.builder.CreateCondBr(isfalse, ifnot, ifso);
             find_next_stmt(cursor + 1);
             continue;
         }
@@ -6332,6 +6335,19 @@ static std::unique_ptr<Module> emit_function(
             // This edge was statically unreachable. Don't codegen it.
             if (!FromBB)
                 continue;
+            // We folded this branch to an unconditional branch, only codegen it once
+            if (cast<BranchInst>(FromBB->getTerminator())->isUnconditional()) {
+                bool found = false;
+                for (size_t j = 0; j < i; ++j) {
+                    size_t j_edge = jl_unbox_long(jl_array_ptr_ref(edges, j));
+                    if (j_edge == edge) {
+                        found = true;
+                        assert(jl_egal(value, jl_array_ptr_ref(values, j)));
+                    }
+                }
+                if (found)
+                    continue;
+            }
 #ifndef JL_NDEBUG
             if (FromBB) {
                 bool found_pred = false;

test/core.jl

@@ -6202,3 +6202,20 @@ function test27566(a,b)
 end
 test27566(a, b, c, d) = a.*(b, c, d)
 @test test27566(1,1) == (1,0,1)
+
+# Issue #27594
+struct Iter27594 end
+Base.iterate(::Iter27594) = (1, nothing)
+Base.iterate(::Iter27594, ::Any) = nothing
+
+function foo27594()
+    ind = 0
+    for x in (1,)
+        for y in Iter27594()
+            ind += 1
+        end
+    end
+    ind
+end
+
+@test foo27594() == 1