https://bugs.llvm.org/show_bug.cgi?id=48255
Bug ID: 48255
Summary: [AArch64] Using ".byte" from asm in a switch statement
causes bad jump table
Product: clang
Version: trunk
Hardware: PC
OS: MacOS X
Status: NEW
Severity: normal
Priority: P
Component: LLVM Codegen
Assignee: unassignedclangb...@nondot.org
Reporter: myriac...@gmail.com
CC: llvm-bugs@lists.llvm.org, neeil...@live.com,
richard-l...@metafoo.co.uk
Created attachment 24196
--> https://bugs.llvm.org/attachment.cgi?id=24196&action=edit
Compile me with -O2 on ARM64
If you use .byte from within an ARM64 "asm" block to exceed 4 bytes per line, a
"switch" statement could generate a bad jump table, causing unpredictable
behavior. This happens with -O1 and higher, but is more likely with -O2 and
higher.
https://gcc.godbolt.org/z/bGdqb3
The correct output is 1 (i.e. 15 - 14), but instead it prints 29 on ARM64 with
-O2 because the CPU ends up riding the NOP sled.
What happens is that LLVM with -O2 or higher will use a byte-sized table to
store a count of instructions to skip for that entry of the jump table. When
LLVM inlines Meow, this exceeds the values that can be stored in this jump
table, truncating the jump distance to a byte.
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