https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102150
Bug ID: 102150
Summary: Speculative execution of inline assembly causes divide
error
Product: gcc
Version: 11.2.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: rtl-optimization
Assignee: unassigned at gcc dot gnu.org
Reporter: jeremy-gcc-bugzilla at sawicki dot us
Target Milestone: ---
Created attachment 51391
--> https://gcc.gnu.org/bugzilla/attachment.cgi?id=51391&action=edit
Reproducible test case
The attached test case uses inline assembly to wrap the x86_64 DIV instruction.
GCC speculatively executes the inline assembly on inputs that the source
program does not, resulting in a divide error.
The GCC documentation says that non-volatile inline assembly may be discarded
or moved out of loops. It is not obvious whether speculative execution is also
permitted. I asked on gcc-help and was asked to file a report.
A related report points out that many projects currently wrap the DIV
instruction without using volatile:
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=82677
Another related report considers the similar issue of whether pure/const
functions must be non-trapping for inputs they don't actually receive:
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=93491
If it is determined that volatile is required, it would helpful to clarify in
the documentation that speculative execution may occur without volatile:
https://gcc.gnu.org/onlinedocs/gcc/Extended-Asm.html#Volatile
gcc version 11.2.0 (GCC)
Target: x86_64-pc-linux-gnu
Configured with: /home/jeremys/gcc-11.2.0/configure
--prefix=/home/jeremys/gcc-11.2.0-install --disable-multilib
Command line: g++ -O3 -o divasm divasm.cpp
No compiler errors/warnings are produced
When executed, a divide error occurs
