Hi, There:
Clang fails to compile 254.gap @ CPU2000int suite. The symptom is
that executable fail to run
with reference input.
The root cause is that the compiler mistakenly optimizes expr "x * y
/ y" into x where the x*y is blindly
flagged with nsw without any analysis.
The preproceeded code is excerpted bellow:
cat -n integer.i
---------------------------------
2361 TypHandle ProdInt ( hdL, hdR )
2362 TypHandle hdL, hdR;
2363 {
....
2373 if ( (int)hdL & (int)hdR & 1 ) {
2374
2375
2376 i = ((int)hdL - 1) * ((int)hdR >> 1); /* !!! No barbaric
NSW, please !!! */
2377 if ( ((int)hdR >> 1) == 0 || i / ((int)hdR >> 1) ==
((int)hdL-1) ) {
---------------------------------
I catch the case where the multiplication overflow: the 1st and 2nd
operand hold the
value 0x4000 and 0x20000, respectively.
Dose gap source code has some "undefined" behavior according to
some
spec
on this planet? Or it is compiler's fault for blindly mark any mul NSW?
Thanks
Shuxin
This has come up before, and we just added -fwrapv to work around the problem: http://lists.cs.uiuc.edu/pipermail/llvm-commits/Week-of-Mon-20110131/115969.html Are you compiling without -fwrapv? Cameron On May 6, 2013, at 4:55 PM, Shuxin Yang <shuxin.llvm at gmail.com> wrote:> Hi, There: > > Clang fails to compile 254.gap @ CPU2000int suite. The symptom is that executable fail to run > with reference input. > > The root cause is that the compiler mistakenly optimizes expr "x * y / y" into x where the x*y is blindly > flagged with nsw without any analysis. > > The preproceeded code is excerpted bellow: > > cat -n integer.i > --------------------------------- > 2361 TypHandle ProdInt ( hdL, hdR ) > 2362 TypHandle hdL, hdR; > 2363 { > .... > 2373 if ( (int)hdL & (int)hdR & 1 ) { > 2374 > 2375 > 2376 i = ((int)hdL - 1) * ((int)hdR >> 1); /* !!! No barbaric NSW, please !!! */ > 2377 if ( ((int)hdR >> 1) == 0 || i / ((int)hdR >> 1) == ((int)hdL-1) ) { > --------------------------------- > > I catch the case where the multiplication overflow: the 1st and 2nd operand hold the > value 0x4000 and 0x20000, respectively. > > Dose gap source code has some "undefined" behavior according to some spec > on this planet? Or it is compiler's fault for blindly mark any mul NSW? > > Thanks > Shuxin > > > > > > _______________________________________________ > LLVM Developers mailing list > LLVMdev at cs.uiuc.edu http://llvm.cs.uiuc.edu > http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev
On Mon, May 6, 2013 at 4:55 PM, Shuxin Yang <shuxin.llvm at gmail.com> wrote:> Hi, There: > > Clang fails to compile 254.gap @ CPU2000int suite. The symptom is that > executable fail to run > with reference input. > > The root cause is that the compiler mistakenly optimizes expr "x * y / y" > into x where the x*y is blindly > flagged with nsw without any analysis. > > The preproceeded code is excerpted bellow: > > cat -n integer.i > --------------------------------- > 2361 TypHandle ProdInt ( hdL, hdR ) > 2362 TypHandle hdL, hdR; > 2363 { > .... > 2373 if ( (int)hdL & (int)hdR & 1 ) { > 2374 > 2375 > 2376 i = ((int)hdL - 1) * ((int)hdR >> 1); /* !!! No barbaric NSW, > please !!! */ > 2377 if ( ((int)hdR >> 1) == 0 || i / ((int)hdR >> 1) => ((int)hdL-1) ) { > --------------------------------- > > I catch the case where the multiplication overflow: the 1st and 2nd > operand hold the > value 0x4000 and 0x20000, respectively. > > Dose gap source code has some "undefined" behavior according to some spec > on this planet? Or it is compiler's fault for blindly mark any mul NSW?Signed arithmetic overflow is undefined in C and C++. - David
It works like a charm! Thanks a lot! Shuxin On 5/6/13 4:59 PM, Cameron Zwarich wrote:> This has come up before, and we just added -fwrapv to work around the problem: > > http://lists.cs.uiuc.edu/pipermail/llvm-commits/Week-of-Mon-20110131/115969.html > > Are you compiling without -fwrapv? > > Cameron > > On May 6, 2013, at 4:55 PM, Shuxin Yang <shuxin.llvm at gmail.com> wrote: > >> Hi, There: >> >> Clang fails to compile 254.gap @ CPU2000int suite. The symptom is that executable fail to run >> with reference input. >> >> The root cause is that the compiler mistakenly optimizes expr "x * y / y" into x where the x*y is blindly >> flagged with nsw without any analysis. >> >> The preproceeded code is excerpted bellow: >> >> cat -n integer.i >> --------------------------------- >> 2361 TypHandle ProdInt ( hdL, hdR ) >> 2362 TypHandle hdL, hdR; >> 2363 { >> .... >> 2373 if ( (int)hdL & (int)hdR & 1 ) { >> 2374 >> 2375 >> 2376 i = ((int)hdL - 1) * ((int)hdR >> 1); /* !!! No barbaric NSW, please !!! */ >> 2377 if ( ((int)hdR >> 1) == 0 || i / ((int)hdR >> 1) == ((int)hdL-1) ) { >> --------------------------------- >> >> I catch the case where the multiplication overflow: the 1st and 2nd operand hold the >> value 0x4000 and 0x20000, respectively. >> >> Dose gap source code has some "undefined" behavior according to some spec >> on this planet? Or it is compiler's fault for blindly mark any mul NSW? >> >> Thanks >> Shuxin >> >> >> >> >> >> _______________________________________________ >> LLVM Developers mailing list >> LLVMdev at cs.uiuc.edu http://llvm.cs.uiuc.edu >> http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev