Author: hans
Date: Thu Aug 29 05:19:19 2019
New Revision: 370342
URL: http://llvm.org/viewvc/llvm-project?rev=370342&view=rev
Log:
ReleaseNotes: omitting range checks for switches with unreachable defaults
Modified:
llvm/branches/release_90/docs/ReleaseNotes.rst
Modified: llvm/branches/release_90/docs/ReleaseNotes.rst
URL:
http://llvm.org/viewvc/llvm-project/llvm/branches/release_90/docs/ReleaseNotes.rst?rev=370342&r1=370341&r2=370342&view=diff
==============================================================================
--- llvm/branches/release_90/docs/ReleaseNotes.rst (original)
+++ llvm/branches/release_90/docs/ReleaseNotes.rst Thu Aug 29 05:19:19 2019
@@ -85,6 +85,36 @@ Noteworthy optimizations
`bug 42763 <https://bugs.llvm.org/show_bug.cgi?id=42763>_` and
`post commit discussion
<http://lists.llvm.org/pipermail/llvm-commits/Week-of-Mon-20190422/646945.html>_`.
+* LLVM will now omit range checks for jump tables when lowering switches with
+ unreachable default destination. For example, the switch dispatch in the C++
+ code below
+
+ .. code-block:: c
+
+ int g(int);
+ enum e { A, B, C, D, E };
+ int f(e x, int y, int z) {
+ switch(x) {
+ case A: return g(y);
+ case B: return g(z);
+ case C: return g(y+z);
+ case D: return g(x-z);
+ case E: return g(x+z);
+ }
+ }
+
+ will result in the following x86_64 machine code when compiled with Clang.
+ This is because falling off the end of a non-void function is undefined
+ behaviour in C++, and the end of the function therefore being treated as
+ unreachable:
+
+ .. code-block:: asm
+
+ _Z1f1eii:
+ mov eax, edi
+ jmp qword ptr [8*rax + .LJTI0_0]
+
+
Changes to the LLVM IR
----------------------
_______________________________________________
llvm-branch-commits mailing list
llvm-branch-commits@lists.llvm.org
https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-branch-commits
