Does loop operation with variable assignment violate SSA principle?

Question

I just started to learn LLVM IR and SSA, got a question about the SSA principle. I found the following code block on the Internet, which seems to violate SSA principle because variables are assigned value for several times. Is my comprehension right?

; <label>:4:                                     ; preds = %7, %0 
  %5 = load i32, i32* %3, align 4
  %6 = icmp slt i32 %5, 10
  br i1 %6, label %7, label %12

; <label>:7:                                      ; preds = %4  
  %8 = load i32, i32* %3, align 4
  %9 = add nsw i32 %8, 1 
  store i32 %9, i32* %3, align 4
  %10 = load i32, i32* %2, align 4
  %11 = mul nsw i32 %10, 2 
  store i32 %11, i32* %2, align 4
  br label %4

score 3 · Answer 1 · answered Jun 10 '22 at 05:12

LLVM uses "partial SSA" form. LLVM's infinite registers are in SSA form but memory and global variables are not. Your %5 can take on different values because it is a load from memory.

Even in fully SSA form an SSA value in a loop ordinarily takes on different values through the loop iterations. It would look like %5 = phi i32 [%start_val, %loopheader_bb], [%iteration_val, %backedge_bb]. You should get phi nodes if you run opt -sroa over your code.

Does loop operation with variable assignment violate SSA principle?

1 Answers1