opt -O2 generates different results

[llvmbot]

Bugzilla Link	11453
Resolution	INVALID
Resolved on	Jan 27, 2014 16:08
Version	trunk
OS	Linux
Attachments	Test program to reproduce the bug, Test program llvm/llvm-project#374 to reproduce the bug
Reporter	LLVM Bugzilla Contributor
CC	@zygoloid

Extended Description

1. On ARM, the output of the attached programs is:
   Min is negative
   Min is negative
   Min is negative
   Min is negative

2. But if "-O2" is used, on ARM the binary generates different results:
   Min is negative
   Min is negative
   Min is negative
   Min is NOT negative

To reproduce "1" above, please do:
A. cpp to ll
~/llvmUpstream/Release+Asserts/bin/clang -o lib.ll -S -emit-llvm lib.cpp
B. ll to bc
~/llvmUpstream/Release+Asserts/bin/llvm-as -o=lib.bc lib.ll
C. bc to o
~/llvmUpstream/Release+Asserts/bin/llc -relocation-model=pic -o=lib.o -filetype=obj -mtriple=armv7-none-linux-gnueabi -mattr=+v7 -float-abi=soft ./lib.bc
D. o to so
/Android/prebuilt/linux-x86/toolchain/arm-linux-androideabi-4.4.x/bin/arm-linux-androideabi-ld -soname libtest.so -L /Android/out/target/product/maguro/system/lib -lc -shared --no-undefined --fix-cortex-a8 -rpath-link=/Android/out/target/product/maguro/system/lib -rpath=/Android/out/target/product/maguro/system/lib -freloc=PIC -o ./libtest.so ./lib.o ~/Android/prebuilt/linux-x86/toolchain/arm-linux-androideabi-4.4.x/lib/gcc/arm-linux-androideabi/4.4.3/libgcc.a

To reproduce "2" above, please insert Step B1 and B2 while issuing the commands A-D above.

B1. Use opt with -O2. bc to bc.
~/llvmUpstream/Release+Asserts/bin/opt -o=lib.opt.bc -O2 lib.bc
B2. mv lib.opt.bc lib.bc

[llvmbot]

I think the nsw (no signed wrapper) claim added by the frontend may be at the issue here. The signed case is undefined in the C world, unlike the unsigned case.

I think Dan Gohman and Logan Chien would categorize this bug as undefined behavior and the upstream may not consider this as a bug...

Converting "-x < 0" to "x >= 0" is valid for "opt -O2". When there is nsw, the negation is well defined in [- (2^63 - 1), (2^63 -1)].

Logan was suggesting to me to change:

%sub = sub nsw i64 0, %0

to

%sub = sub i64 0, %0

[zygoloid]

The test case has undefined behavior, and this result is expected. You can use -fwrapv to stop the 'nsw' flag being emitted.