Now that icmp and fcmp have supported returning vectors of i1 for a while, I think it's time to remove the vicmp and vfcmp instructions from LLVM. The good news is that we've never shipped a release that included them so we won't be providing auto-upgrade support. There is some existing backend support for vicmp and vfcmp that looks different from what icmp and fcmp do. If this actually matters to you, please port the bits you need over to icmp/fcmp, or let me know if you think you still need vicmp/vfcmp and can't switch within (say) a week. If I don't hear from anyone within a week then I'll go ahead and rip the existing support out. Thanks! Nick
Mai, Haohui
2009-Jun-24 05:41 UTC
[LLVMdev] Handling SMax(N, N - constInt) in Scalar Evolution pass
Hi all, I'm working on a project which tries to prove an access to an array is safe. For example, int foo(int s) { int * p = malloc(s * sizeof int); ... int q = p[s - 2]; } then the access of p[s - 2] always stays in bound. I implemented a prototype using the Scalar Evolution pass. Here are the pseudo-code of the implementation: const SCEV * offset = SE->getMinusSCEV(SE->getSCEV(GEP), GEPBase); const SCEV * bounds = SE->getSCEV(objSize); if (SE->getSMaxExpr(offset, bounds) == bounds) { ++safeGEPs; } But it turns out that SCEVSMaxExpr does not handle the case of SMax(N, N-2). My question is, is there a plan to support something like this, or is it possible to do some refactoring to enhance the capability of Scalar Evolution? I notice that Scalar Evolution, Instruction Combining and Memory Dependence Analysis require sort of evaluating symbolic expression like this. For this case SMax(A, B) is equivalent to SMax(A-B,0) + B, instruction combining handles sophisticated expressions like (A+B)-B pretty well. It would be great if Scalar Evolution can support this. Any comments are appreciated. Cheers, Haohui
Nick Lewycky
2009-Jun-24 05:55 UTC
[LLVMdev] Handling SMax(N, N - constInt) in Scalar Evolution pass
Mai, Haohui wrote:> Hi all, > > I'm working on a project which tries to prove an access to an array is > safe. For example, > > int foo(int s) { > int * p = malloc(s * sizeof int); > ... > int q = p[s - 2]; > } > > then the access of p[s - 2] always stays in bound. > > I implemented a prototype using the Scalar Evolution pass. Here are the > pseudo-code of the implementation: > > const SCEV * offset = SE->getMinusSCEV(SE->getSCEV(GEP), GEPBase); > const SCEV * bounds = SE->getSCEV(objSize); > > if (SE->getSMaxExpr(offset, bounds) == bounds) { > ++safeGEPs; > } > > But it turns out that SCEVSMaxExpr does not handle the case of SMax(N, > N-2).Consider 8-bit integers and N = -127. N-1 equals INT_MIN and N-2 then is equal to INT_MAX, or 127. In that case, the SMax would equal N-2, not N. In other cases (like N = 2) the SMax would equal N, not N-2. Because of this, we cannot reduce this SMax any further. Your suggestion that "SMax(A, B) == SMax(A-B, 0) + B" is incorrect. Nick> My question is, is there a plan to support something like this, or is it > possible to do some refactoring to enhance the capability of Scalar > Evolution? I notice that Scalar Evolution, Instruction Combining and > Memory Dependence Analysis require sort of evaluating symbolic > expression like this. > > For this case SMax(A, B) is equivalent to SMax(A-B,0) + B, instruction > combining handles sophisticated expressions like (A+B)-B pretty well. > It would be great if Scalar Evolution can support this. > > Any comments are appreciated. > > Cheers, > > Haohui > > _______________________________________________ > LLVM Developers mailing list > LLVMdev at cs.uiuc.edu http://llvm.cs.uiuc.edu > http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev >
Hi Nick,> Now that icmp and fcmp have supported returning vectors of i1 for a > while,the code generators don't know how to codegen vectors of i1, so does this actually work? Ciao, Duncan.
On Jun 24, 2009, at 12:47 AM, Duncan Sands wrote:> Hi Nick, > >> Now that icmp and fcmp have supported returning vectors of i1 for a >> while, > > the code generators don't know how to codegen vectors of i1, so does > this actually work?No, but there are no clients of them yet. -Chris
Seemingly Similar Threads
- [LLVMdev] Handling SMax(N, N - constInt) in Scalar Evolution pass
- [LLVMdev] Handling SMax(N, N - constInt) in Scalar Evolution pass
- [LLVMdev] Handling SMax(N, N - constInt) in Scalar Evolution pass
- [LLVMdev] Handling SMax(N, N - constInt) in Scalar Evolution pass
- [LLVMdev] Handling SMax(N, N - constInt) in Scalar Evolution pass