Hi all,


this a question regarding the Compiler's implementation. I spent some time on it but could not yet find a satisfying explanation. Maybe someone of you can help me?


Let's look into MessageNode >> #emitCodeForCase:encoder:value:, which contains the following passage:


(isLast and: [allReturn]) ifTrue:

    [self emitCodeForJump: elseSize encoder: encoder]


Btw, the equivalent in #sizeCodeForCase:value: is:


(isLast and: [allReturn]) ifTrue: [thenSize := thenSize + (self sizeCode: encoder forJump: elseSize)].


In the bytecode of a method, this will look like this:


89 <23> pushConstant: 2

90 <88> dup

91 <76> pushConstant: 1

92 <B6> send: =

93 <9A> jumpFalse: 97

94 <87> pop

95 <22> pushConstant: #one

96 <7C> returnTop

97 <77> pushConstant: 2

98 <B6> send: =

99 <9A> jumpFalse: 103

100 <21> pushConstant: #two

101 <7C> returnTop

102 <90> jumpTo: 104 "here!"

103 <20> pushConstant: 42

104 <87> pop


I cannot see when this jump would be ever reached.

Case 1: Any case is selected, and because allReturn is true, its inlined value block is guaranteed to #returnTop and thus not to reach the last jump (in the above example, this happens in pc101).

Case 2: No case is selected, so the execution will jump straightly to the inlined otherwiseBlock (this is pc99 in the example).

For what purpose do we need this jump (ex: pc102)? Should we maybe remove it?


Are there any special situations in which #returns is true but there is any code path that does *not* return? Some weird exception handling trick, for example?

Or was the dubious jump just designed as a fallback to catch any hypothetic failures in any other component of the compiler system?


I removed the generation of this jump for testing and could not investigate any anomalies so far. All compiler tests are passing - except of a number of Decompiler tests, which probably would need to be updated as well.

Please find a minimum changeset in the attachment that demonstrates my question.


I am looking forward to your replies!


Best,

Christoph
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