Class Conditional
- java.lang.Object
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- org.apache.derby.impl.services.bytecode.Conditional
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class Conditional extends java.lang.Object
A Conditional represents an if/then/else block. When this is created the code will already have the conditional check code. The code is optimized for branch offsets that fit in 2 bytes, though will handle 4 byte offsets.if condition then code else code
what actually gets built isif !condition branch to eb: then code goto end: // skip else eb: else code end:
If no else condition was provided then the code is:if !condition branch to end: then code end:
Note all branches here are using relative offsets, not absolute program counters. If the then code leads to the conditional branch offset being too big (>32k) because the then code is larger than 32767 bytes then this is built:// when else code is present if condition branch to tb: (relative offset +8) goto_w eb: // indirect for else block (5 bytes) tb: then code (> 32767 bytes) goto end: eb: else code end:
// when only then code is present if condition branch to tb: (relative offset +8) goto_w end: // indirect for else block (5 bytes) tb: then code (> 32767 bytes) end:
If there is an else branch and only it is larger than 32767 bytes then the code is:if !condition branch to eb: (offset increased by two over previous value) then code goto_w end: // skip else eb: else code (> 32767 bytes) end:
This has one special case where the size of conditional branch to eb: now must change from a 16bit value to a 32 bit value. The generated code for this is the same as when both the then code and the else code require 32bit offsets for the branches. This code is:if condition branch to tb: (relative offset +8) goto_w eb: // indirect for else block (5 bytes) tb: then code (> 32767 bytes) goto_w end: eb: else code (> 32767 bytes) end:
In theory, at the moment this should not happen as this would mean a total code size that exceeds the limit on the code size for a method (64k). This code handles this case as it does occur if the limit for a branch is lowered for testing purposes, to ensure the complete set of branch re-write code works. This lowering of the limit can be done by changing the constant BRANCH16LIMIT.
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Field Summary
Fields Modifier and Type Field Description private static int
BRANCH16LIMIT
Limit of a 16 bit branch.private int
if_pc
pc of the 'if' opcode.private Conditional
parent
private Type[]
stack
private int
thenGoto_pc
pc of the GOTO added at the end of the then block to transfer control to the end of this conditional.
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Constructor Summary
Constructors Constructor Description Conditional(Conditional parent, CodeChunk chunk, short ifOpcode, Type[] entryStack)
Start a conditional block.
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Method Summary
All Methods Instance Methods Concrete Methods Modifier and Type Method Description (package private) Conditional
end(BCMethod mb, CodeChunk chunk, Type[] elseStack, int stackNumber)
Complete the conditional and patch up any jump instructions.private void
fillIn(BCMethod mb, CodeChunk chunk, int branch_pc, int target_pc)
Fill in the offsets for a conditional or goto instruction that were dummied up as zero during code generation.(package private) Type[]
startElse(BCMethod mb, CodeChunk chunk, Type[] thenStack)
Complete the 'then' block and start the 'else' block for this conditional
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Field Detail
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BRANCH16LIMIT
private static final int BRANCH16LIMIT
Limit of a 16 bit branch.If broad testing of the switch from 16bit to 32bit offsets is required then this constant can be reduced to a lower value, say 50 and run complete tests. This will cover all the combinations. This works because the GOTO_W instruction works with any offset value.
- See Also:
- Constant Field Values
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parent
private final Conditional parent
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if_pc
private final int if_pc
pc of the 'if' opcode.
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stack
private Type[] stack
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thenGoto_pc
private int thenGoto_pc
pc of the GOTO added at the end of the then block to transfer control to the end of this conditional. That is at the end of the else block.
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Constructor Detail
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Conditional
Conditional(Conditional parent, CodeChunk chunk, short ifOpcode, Type[] entryStack)
Start a conditional block.- Parameters:
parent
- Current conditional block, null if no nesting is going on.chunk
- CodeChunk this conditional lives inifOpcode
- Opcode for the if check.entryStack
- Type stack on entering the conditional then block.
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Method Detail
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startElse
Type[] startElse(BCMethod mb, CodeChunk chunk, Type[] thenStack)
Complete the 'then' block and start the 'else' block for this conditional- Parameters:
chunk
- CodeChunk this conditional lives inthenStack
- Type stack on completing the conditional then block.- Returns:
- the type stack on entering the then block
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end
Conditional end(BCMethod mb, CodeChunk chunk, Type[] elseStack, int stackNumber)
Complete the conditional and patch up any jump instructions.- Parameters:
chunk
- CodeChunk this conditional lives inelseStack
- Current stack, which is the stack at the end of the elsestackNumber
- Current number of valid elements in elseStack- Returns:
- The conditional this conditional was nested in, if any.
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fillIn
private void fillIn(BCMethod mb, CodeChunk chunk, int branch_pc, int target_pc)
Fill in the offsets for a conditional or goto instruction that were dummied up as zero during code generation. Handles modifying branch logic when the offset for the branch is greater than can fit in 16 bits. In this case a GOTO_W with a 32 bit offset will be used, see details within the method for how this is acheived in all situations. This method might insert instructions in the already generated byte code, thus increasing the program counter.- Parameters:
mb
- Method this conditional is forchunk
- Our code chunkbranch_pc
- pc of the branch or goto opcode in the code streamtarget_pc
- pc where we want to jump to.
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