The Ignition statement list visitor will skip the rest of the
statements in the list if it hits a jump statement (like a return
or break), as the rest of the code in the list can be considered
dead.
return;
dead_call(); // skipped
However, since this is at an AST node level, it does not take into
account condition shortcutting:
if(2.2) return;
dead_call(); // not skipped
There is also a second dead code elimination in Ignition compilation, at
the bytecode array writer level, where a bytecodes are not emitted if an
"exit" bytecode (Return, Jump, or a few others) has been written, until
the next basic block starts (i.e. a Bind).
This can cause an issue with statements that resurrect the bytecode
array writer part-way through their visit. An example is try-catch
statements, which save the context to a register, and then Bind to start
the try region.
For the case:
if (2.2) return;
try { // try statement not skipped
...
}
the bytecode writer is called with
OutputReturn() // exit bytecode seen
OutputMove(<context>, r1) // not emitted
Bind(&try_begin) // starts new basic block
// try body
So, the try is emitted, but without saving the context to a register.
This means that the liveness analysis sees the read of that register
(as the output liveness of throwing bytecodes), but does not have a
write to the register, which means that the liveness escapes.
This patch fixes this by using the bytecode array writer dead-code
elimination (i.e. "exit bytecode seen") to inform the statement list
visitor, so that in this example the try statement is not visited at
all.
Bug: chromium:902395
Change-Id: Ieb8e46a4318df3edbac0ae17235e0ce8fba12ee3
Reviewed-on: https://chromium-review.googlesource.com/c/1322951
Reviewed-by: Mythri Alle <mythria@chromium.org>
Commit-Queue: Leszek Swirski <leszeks@chromium.org>
Cr-Commit-Position: refs/heads/master@{#57350}
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