Bug 193304

Summary: DFG combined liveness can be wrong for terminal basic blocks
Product: WebKit Reporter: Saam Barati <saam>
Component: JavaScriptCoreAssignee: Saam Barati <saam>
Status: RESOLVED FIXED    
Severity: Normal CC: benjamin, commit-queue, fpizlo, ggaren, gskachkov, keith_miller, mark.lam, msaboff, rmorisset, ticaiolima, webkit-bug-importer, ysuzuki
Priority: P2 Keywords: InRadar
Version: WebKit Nightly Build   
Hardware: Unspecified   
OS: Unspecified   
Attachments:
Description Flags
patch
ysuzuki: review+
patch for landing none

Description Saam Barati 2019-01-09 15:00:02 PST 
If a block doesn't have any successors, it can't rely on the typical backwards liveness propagation that CombinedLiveness was doing.
For such terminal blocks, we must account for what's bytecode live at the end of the block. Consider a block that ends with:
```
ForceOSRExit
Unreachable
```

Things may definitely be live in bytecode at the tail. However, we'll report nothing as being alive. This probably subtly breaks many analyses, but we have a test case of it breaking the interference analysis that the ArgumentsEliminationPhase performs.
Comment 1 Saam Barati 2019-01-09 15:00:46 PST 
<rdar://problem/45268632>
Comment 2 Saam Barati 2019-01-09 15:05:36 PST 
Created attachment 358753 [details]
patch
Comment 3 Yusuke Suzuki 2019-01-10 15:00:11 PST 
Comment on attachment 358753 [details]
patch

r=me
Comment 4 Saam Barati 2019-01-11 15:13:42 PST 
(In reply to Yusuke Suzuki from comment #3)
> Comment on attachment 358753 [details]
> patch
> 
> r=me

Thanks for the review. Yusuke and I also spoke offline, and I'll make this patch fix some errors in a comment in the CombinedLiveness header.
Comment 5 Saam Barati 2019-01-11 15:18:29 PST 
Created attachment 358955 [details]
patch for landing
Comment 6 WebKit Commit Bot 2019-01-11 16:26:12 PST 
Comment on attachment 358955 [details]
patch for landing

Clearing flags on attachment: 358955

Committed r239882: <https://trac.webkit.org/changeset/239882>
Comment 7 WebKit Commit Bot 2019-01-11 16:26:13 PST 
All reviewed patches have been landed.  Closing bug.