Zhao, Weiming via llvm-dev
2016-Mar-17 01:16 UTC
[llvm-dev] How to let LLVM handle undefined behavior more gracefully?
Hi,
There are cases where LLVM is able to detect some UB but clang is not.
For example,
unsigned int foo(unsigned int x) {
int ret = 0;
for(int i = 0; i <= 32; ++i)
ret += x >> i;
return ret;
}
When the loop is unrolled, LLVM InstructionSimplify will catch it and
return a UNDEF value.
How can we let LLVM report some warning message to help developers
correct the error?
Or should we use similar behavior as GCC (e.g. x >> 32 returns 0)?
This can also saves compiler engineer's effort: users complain that it's
a compiler bug because their code works with GCC or older version of
LLVM (because the loop is not unrolled). And it's really hard to debug
such UB in some large code base.
Thanks,
Weiming
--
Qualcomm Innovation Center, Inc. is a member of Code Aurora Forum, hosted by The
Linux Foundation
Mehdi Amini via llvm-dev
2016-Mar-17 01:29 UTC
[llvm-dev] How to let LLVM handle undefined behavior more gracefully?
> On Mar 16, 2016, at 6:16 PM, Zhao, Weiming via llvm-dev <llvm-dev at lists.llvm.org> wrote: > > Hi, > > There are cases where LLVM is able to detect some UB but clang is not. > > For example, > > unsigned int foo(unsigned int x) { > > int ret = 0; > for(int i = 0; i <= 32; ++i) > ret += x >> i; > return ret; > > } > > When the loop is unrolled, LLVM InstructionSimplify will catch it and > return a UNDEF value. > How can we let LLVM report some warning message to help developers > correct the error? > Or should we use similar behavior as GCC (e.g. x >> 32 returns 0)? > > This can also saves compiler engineer's effort: users complain that it's > a compiler bug because their code works with GCC or older version of > LLVM (because the loop is not unrolled). And it's really hard to debug > such UB in some large code base.Isn't it catched by UBSAN? -- Mehdi> > Thanks, > Weiming > > -- > Qualcomm Innovation Center, Inc. is a member of Code Aurora Forum, hosted by The Linux Foundation > > _______________________________________________ > LLVM Developers mailing list > llvm-dev at lists.llvm.org > http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
via llvm-dev
2016-Mar-17 04:42 UTC
[llvm-dev] How to let LLVM handle undefined behavior more gracefully?
Yes, thought of UBSan. But in our case, the target program runs on baremetal. It has very tight code size restriction and it has no stderr. Since LLVM already caught the behavior during compilation, it should notify users about it. On 2016-03-16 18:29, Mehdi Amini wrote:>> On Mar 16, 2016, at 6:16 PM, Zhao, Weiming via llvm-dev >> <llvm-dev at lists.llvm.org> wrote: >> >> Hi, >> >> There are cases where LLVM is able to detect some UB but clang is not. >> >> For example, >> >> unsigned int foo(unsigned int x) { >> >> int ret = 0; >> for(int i = 0; i <= 32; ++i) >> ret += x >> i; >> return ret; >> >> } >> >> When the loop is unrolled, LLVM InstructionSimplify will catch it and >> return a UNDEF value. >> How can we let LLVM report some warning message to help developers >> correct the error? >> Or should we use similar behavior as GCC (e.g. x >> 32 returns 0)? >> >> This can also saves compiler engineer's effort: users complain that >> it's >> a compiler bug because their code works with GCC or older version of >> LLVM (because the loop is not unrolled). And it's really hard to debug >> such UB in some large code base. > > Isn't it catched by UBSAN? > > -- > Mehdi > > >> >> Thanks, >> Weiming >> >> -- >> Qualcomm Innovation Center, Inc. is a member of Code Aurora Forum, >> hosted by The Linux Foundation >> >> _______________________________________________ >> LLVM Developers mailing list >> llvm-dev at lists.llvm.org >> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev