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author | Lukas Rytz <lukas.rytz@gmail.com> | 2016-11-08 11:58:01 +0100 |
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committer | Lukas Rytz <lukas.rytz@gmail.com> | 2016-11-08 11:58:01 +0100 |
commit | f297ca8d1f06086316ff3746250092e36ef9f74e (patch) | |
tree | 90755ce5098f1f6fee54d7611ae24311646a0822 /src | |
parent | 10c609e750a7089055b126e6231e5ddb2f2e8623 (diff) | |
download | scala-f297ca8d1f06086316ff3746250092e36ef9f74e.tar.gz scala-f297ca8d1f06086316ff3746250092e36ef9f74e.tar.bz2 scala-f297ca8d1f06086316ff3746250092e36ef9f74e.zip |
SI-10032 Fix code gen with returns in nested try-finally blocks
Return statements within `try` or `catch` blocks need special treatement
if there's also a `finally`
try { return 1 } finally { println() }
For the return, the code generator emits a store to a local and a jump
to a "cleanup" version of the finally block. There will be 3 versions
of the finally block:
- One reached through a handler, if the code in the try block
throws; re-throws at the end
- A "cleanup" version reached from returns within the try; reads the
local and returns the value at the end
- One reached for ordinary control flow, if there's no return and no
exception within the try
If there are multiple enclosing finally blocks, a "cleanup" version is
emitted for each of them. The nested ones jump to the enclosing ones,
the outermost one reads the local and returns.
A global variable `shouldEmitCleanup` stores whether cleanup versions
are required for the curren finally blocks. By mistake, this variable
was not reset to `false` when emitting a `try-finally` nested within a
`finally`:
try {
try { return 1 }
finally { println() } // need cleanup version
} finally { // need cleanup version
try { println() }
finally { println() } // no cleanup version needed!
}
In this commit we ensure that the variable is reset when emitting
nested `try-finally` blocks.
Diffstat (limited to 'src')
-rw-r--r-- | src/compiler/scala/tools/nsc/backend/jvm/BCodeSyncAndTry.scala | 45 |
1 files changed, 38 insertions, 7 deletions
diff --git a/src/compiler/scala/tools/nsc/backend/jvm/BCodeSyncAndTry.scala b/src/compiler/scala/tools/nsc/backend/jvm/BCodeSyncAndTry.scala index 466793010f..9d4ef44546 100644 --- a/src/compiler/scala/tools/nsc/backend/jvm/BCodeSyncAndTry.scala +++ b/src/compiler/scala/tools/nsc/backend/jvm/BCodeSyncAndTry.scala @@ -36,7 +36,7 @@ abstract class BCodeSyncAndTry extends BCodeBodyBuilder { // if the synchronized block returns a result, store it in a local variable. // Just leaving it on the stack is not valid in MSIL (stack is cleaned when leaving try-blocks). val hasResult = (expectedType != UNIT) - val monitorResult: Symbol = if (hasResult) locals.makeLocal(tpeTK(args.head), "monitorResult") else null; + val monitorResult: Symbol = if (hasResult) locals.makeLocal(tpeTK(args.head), "monitorResult") else null /* ------ (1) pushing and entering the monitor, also keeping a reference to it in a local var. ------ */ genLoadQualifier(fun) @@ -215,7 +215,7 @@ abstract class BCodeSyncAndTry extends BCodeBodyBuilder { * please notice `tmp` has type tree.tpe, while `earlyReturnVar` has the method return type. * Because those two types can be different, dedicated vars are needed. */ - val tmp = if (guardResult) locals.makeLocal(tpeTK(tree), "tmp") else null; + val tmp = if (guardResult) locals.makeLocal(tpeTK(tree), "tmp") else null /* * upon early return from the try-body or one of its EHs (but not the EH-version of the finally-clause) @@ -238,6 +238,34 @@ abstract class BCodeSyncAndTry extends BCodeBodyBuilder { val endTryBody = currProgramPoint() bc goTo postHandlers + /** + * A return within a `try` or `catch` block where a `finally` is present ("early return") + * emits a store of the result to a local, jump to a "cleanup" version of the `finally` block, + * and sets `shouldEmitCleanup = true` (see [[PlainBodyBuilder.genReturn]]). + * + * If the try-catch is nested, outer `finally` blocks need to be emitted in a cleanup version + * as well, so the `shouldEmitCleanup` variable remains `true` until the outermost `finally`. + * Nested cleanup `finally` blocks jump to the next enclosing one. For the outermost, we emit + * a read of the local variable, a return, and we set `shouldEmitCleanup = false` (see + * [[pendingCleanups]]). + * + * Now, assume we have + * + * try { return 1 } finally { + * try { println() } finally { println() } + * } + * + * Here, the outer `finally` needs a cleanup version, but the inner one does not. The method + * here makes sure that `shouldEmitCleanup` is only propagated outwards, not inwards to + * nested `finally` blocks. + */ + def withFreshCleanupScope(body: => Unit) = { + val savedShouldEmitCleanup = shouldEmitCleanup + shouldEmitCleanup = false + body + shouldEmitCleanup = savedShouldEmitCleanup || shouldEmitCleanup + } + /* ------ (2) One EH for each case-clause (this does not include the EH-version of the finally-clause) * An EH in (2) is reached upon abrupt termination of (1). * An EH in (2) is protected by: @@ -246,8 +274,7 @@ abstract class BCodeSyncAndTry extends BCodeBodyBuilder { * ------ */ - for (ch <- caseHandlers) { - + for (ch <- caseHandlers) withFreshCleanupScope { // (2.a) emit case clause proper val startHandler = currProgramPoint() var endHandler: asm.Label = null @@ -277,9 +304,13 @@ abstract class BCodeSyncAndTry extends BCodeBodyBuilder { protect(startTryBody, endTryBody, startHandler, excType) // (2.c) emit jump to the program point where the finally-clause-for-normal-exit starts, or in effect `after` if no finally-clause was given. bc goTo postHandlers - } + // Need to save the state of `shouldEmitCleanup` at this point: while emitting the first + // version of the `finally` block below, the variable may become true. But this does not mean + // that we need a cleanup version for the current block, only for the enclosing ones. + val currentFinallyBlockNeedsCleanup = shouldEmitCleanup + /* ------ (3.A) The exception-handler-version of the finally-clause. * Reached upon abrupt termination of (1) or one of the EHs in (2). * Protected only by whatever protects the whole try-catch-finally expression. @@ -288,7 +319,7 @@ abstract class BCodeSyncAndTry extends BCodeBodyBuilder { // a note on terminology: this is not "postHandlers", despite appearances. // "postHandlers" as in the source-code view. And from that perspective, both (3.A) and (3.B) are invisible implementation artifacts. - if (hasFinally) { + if (hasFinally) withFreshCleanupScope { nopIfNeeded(startTryBody) val finalHandler = currProgramPoint() // version of the finally-clause reached via unhandled exception. protect(startTryBody, finalHandler, finalHandler, null) @@ -316,7 +347,7 @@ abstract class BCodeSyncAndTry extends BCodeBodyBuilder { // this is not "postHandlers" either. // `shouldEmitCleanup` can be set, and at the same time this try expression may lack a finally-clause. // In other words, all combinations of (hasFinally, shouldEmitCleanup) are valid. - if (hasFinally && shouldEmitCleanup) { + if (hasFinally && currentFinallyBlockNeedsCleanup) { val savedInsideCleanup = insideCleanupBlock insideCleanupBlock = true markProgramPoint(finCleanup) |