JI-9015069

Compiler Bug - Incorrect results are returned in the end (by using for example nested xor-assignment operators)

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Overview and description:
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As this code

int a = 10, b = 20;
a ^= (b ^= (a ^= b));

should swap the values of "a" and "b", actually makes the compiler spit out wrong bytecode,
hence you get different results in the end.

Results:
a = 0
b = 10


Where this method

int a = 10, b = 20;
a ^= b;
b ^= a;
a ^= b;

returns the correct values.

Results:
a = 20
b = 10


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Versions:
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javac.exe version:
javac 1.8.0_20

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java.exe version:
java version "1.8.0_20"
Java(TM) SE Runtime Environment (build 1.8.0_20-b26)
Java HotSpot(TM) 64-Bit Server VM (build 25.20-b23, mixed mode)

--------------------

javap.exe version:
1.8.0_20


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Decompiled bytecode from the incorrect working method (partial):
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; BugSwap.class (partial view of the main() method)

; int a = 10, b = 20;
bipush	10	; Push 10 into stack (Int)
istore_1	; Store into 1st variable (Int)
bipush	20	; Push 20 into stack (Int)
istore_2	; Store into 2nd variable (Int)

; a ^= (b ^= (a ^= b));
iload_1		; Load from 1st variable (Int)
iload_2		; Load from 2nd variable (Int)
iload_1		; Load from 1st variable (Int)
iload_2		; Load from 2nd variable (Int)
ixor		; XOR (Int)
dup			; Duplicate top operand into stack
istore_1	; Store into 1st variable (Int)
ixor		; XOR (Int)
dup			; Duplicate top operand into stack
istore_2	; Store into 2nd variable (Int)
ixor		; XOR (Int)
istore_1	; Store into 1st variable (Int)

; Results:
; a = 0
; b = 10
;
; Incorrect!


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Decompiled bytecode from the correct working method (partial):
------------------------------

; AlternativeSwap.class (partial view of the main() method)

; int a = 10, b = 20;
bipush	10	; Push 10 into stack (Int)
istore_1	; Store into 1st variable (Int)
bipush	20	; Push 20 into stack (Int)
istore_2	; Store into 2nd variable (Int)

; a ^= b;
iload_1		; Load from 1st variable (Int)
iload_2		; Load from 2nd variable (Int)
ixor		; XOR (Int)
istore_1	; Store into 1st variable (Int)

; b ^= a;
iload_2		; Load from 2nd variable (Int)
iload_1		; Load from 1st variable (Int)
ixor		; XOR (Int)
istore_2	; Store into 2nd variable (Int)

; a ^= b;
iload_1		; Load from 1st variable (Int)
iload_2		; Load from 2nd variable (Int)
ixor		; XOR (Int)
istore_1	; Store into 1st variable (Int)

; Results:
; a = 20
; b = 10
;
; Correct!


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Decompiled bytecode and edited from the former incorrect working method (partial):
------------------------------

; EditedBugSwap.class (partial view of the main() method, which is edited)

; int a = 10, b = 20;
bipush	10	; Push 10 into stack (Int)
istore_1	; Store into 1st variable (Int)
bipush	20	; Push 20 into stack (Int)
istore_2	; Store into 2nd variable (Int)

; a ^= (b ^= (a ^= b));
iload_1		; Load from 1st variable (Int)
iload_2		; Load from 2nd variable (Int)
ixor		; XOR (Int)
dup			; Duplicate top operand into stack
istore_1	; Store into 1st variable (Int)
iload_2		; Load from 2nd variable (Int)
ixor		; XOR (Int)
dup			; Duplicate top operand into stack
istore_2	; Store into 2nd variable (Int)
iload_1		; Load from 2nd variable (Int)
ixor		; XOR (Int)
istore_1	; Store into 1st variable (Int)

; Results:
; a = 20
; b = 10
;
; Correct!


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Conclusion:
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Loading the old values in the first place may work correctly,
if the stack values are synced with the variables,
otherwise you may end up using the incorrect values within the stack,
because the variables are being manipulated in runtime,
hence the alternative code (3rd one) works as intended.