GVNHoist illegally hoists div instruction

[MatzeB]

Bugzilla Link	31729
Resolution	FIXED
Resolved on	Mar 02, 2017 08:56
Version	trunk
OS	All
Blocks	#30970
Attachments	C reproducer, llvm ir reproducer
CC	@zmodem,@hfinkel,@hiraditya,@sanjoy,@sebpop

Extended Description

The attached testcase performs a division (by zero) before checking the condition present in the original sourcecode. The problem appears when compiling test-suite/SingleSource/UnitTests/SignlessTypes/rem.c with the -Oz flag.

I reduced the test to a small reproducer and will attach them here. Looking at the -print-after-all output it seems GVNHoist is to blame and the problem goes away with -gvn-max-hoisted=0 for me.

I can reproduce this on a macOS system with:

$ clang -Oz reduced.c
$ ./a.out
Floating point exception: 8

I attached a -O0 -emit-llvm .ll file for convenience but did not test whether that reproduces as well.

[hiraditya]

Fix for the bug: https://reviews.llvm.org/D29092

[llvmbot]

Is this a duplicate of bug #30154 ?

[MatzeB]

As far as I understand this bug only affects -Os and -Oz users, so #30154 is probably not a duplicate.

[MatzeB]

After some more discussion in the fix for this bug, it may very well be the same (and just the currently proposed fix being too narrow).

[hiraditya]

bug 30806 is different. It is related to jump-threading.

[zmodem]

Fix for the bug: https://reviews.llvm.org/D29092

This was committed in r296642.

I'll merge that to 4.0 once it's been in trunk for a bit.

[hiraditya]

Thanks Hans.

[zmodem]

Merged to 4.0 in r296761.