Hi,
The following java code :
public static void aaa() {
int i = 0;
while (i < 5) {
i = i+1;
}
assertEquals(5, i);
}
is compiled into the following bytecode :
public static void aaa();
Code:
0: iconst_0
1: istore_0
2: goto 8
5: iinc 0, 1
8: iload_0
9: iconst_5
10: if_icmplt 5
13: iconst_5
14: iload_0
15: invokestatic #16; //Method assertEquals:(II)V
18: return
We can see 4 basic blocks:
0..5 (first inclusive, second exclusive)
8..13
5..8
13..19
Jato generates the following HIR:
Control Flow Graph:
#: Range
0xa448c10 0..5
0xa448d80 5..8
0xa448de8 13..19
0xa44ac30 8..13
.. so apparently we get the blocks right. The order is strange but ok.
(Annotated by A.H.) High-Level Intermediate Representation:
[bb 0xa448c10]:
STORE: -- initialize i
store_dest: [local int 0]
store_src: [value int 0x0]
GOTO:
goto_target: [bb 0xa44ac30] -- do the comparison
[bb 0xa448d80]:
STORE: -- increase i
store_dest: [local int 0]
store_src:
BINOP:
vm_type: [int]
binary_operator: [add]
binary_left: [local int 0]
binary_right: [value int 0x1]
///HERE HERE HERE HERE HERE///
As you can see, no branching, no nothing. We fall through to the next block
which calls assertEquals, instead of properly branching to the block which does
the comparison.
Had the two blocks below been swapped, we would not need branching.
[bb 0xa448de8]:
EXPRESSION:
expression:
INVOKE:
target_method: [0xa422a10 'jamvm/TestCase.assertEquals(II)V']
args_list:
ARGS_LIST:
args_left:
ARG:
arg_expression: [local int 0]
args_right:
ARG:
arg_expression: [value int 0x5]
VOID_RETURN
[bb 0xa44ac30]:
IF:
if_conditional: -- test whether to continue the loop
BINOP:
vm_type: [int]
binary_operator: [lt]
binary_left: [local int 0]
binary_right: [value int 0x5]
if_true: [bb 0xa448d80] -- continue looping.
Conclusion: either basic blocks are not ordered correctly, or we need a few
more GOTOs.
Help appreciated.
--
Greetings,
A.H.
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