similar to: LangRef semantics for shufflevector with undef mask is incorrect

Ok, makes sense. My suggestion is that we patch the IR Verifier to ensure that the mask is indeed a vector of constants and/or undefs. Right now it only runs the standard checks for instructions. We will also run Alive2 on the test suite to make sure undef is never replaced in practice. Thanks, Nuno -----Original Message----- From: Eli Friedman <efriedma at quicinc.com> Sent: 27 de

On 11/27/19 2:10 AM, Eli Friedman via llvm-dev wrote: The shuffle mask of a shufflevector is special: it's required to be a constant in a specific form. From LangRef: "The shuffle mask operand is required to be a constant vector with either constant integer or undef values." So really, we can resolve this any way we want; "undef" in this context doesn't have to mean

Hi, Lately we have come to realize how undef makes our life complicated.. Therefore in this email we propose to change the behavior of a few instruction to yield poison instead of undef in error cases. This follows the suggestion of Eli in https://reviews.llvm.org/D33654. Why is undef so bad? - I believe it's not possible to make newgvn correct with undef. See for example the discussion

Sanjay, I'm looking at some missed optimizations caused by D70246. Here's a test case: define <4 x float> @f(i32 %t32, <4 x float>* %t24) { .entry: %t43 = insertelement <3 x i32> undef, i32 %t32, i32 2 %t44 = bitcast <3 x i32> %t43 to <3 x float> %t45 = shufflevector <3 x float> %t44, <3 x float> undef, <4 x i32> <i32 0, i32 undef,

I think calling the attribute "used" is confusing. I'd suggest the following: "not_poison": If an argument is marked not_poison, and the argument is poison at runtime, the call is instant UB. Whether an argument is poison is checked after the rules for other attributes like "nonnull" and "align" are applied. This makes it clear that the IR semantics

Hello all, This is a proposal for reducing # of ptrtoint/inttoptr casts which are not written by programmers but rather generated by LLVM passes. Currently the majority of ptrtoint/inttoptr casts are generated by LLVM; when compiling SPEC 2017 with LLVM r348082 (Dec 2 2018) with -O3, the output IR contains 22,771 inttoptr instructions. However, when compiling it with -O0, there are only 1048

Thanks, Sanjay, that makes sense. The opportunity for improving instcombining splat sounds promising. Another question about shuffle simplification. This is a testcase from test/Transforms/InstCombine/vec_shuffle.ll: define <4 x i32> @test10(<4 x i32> %tmp5) nounwind { %tmp6 = shufflevector <4 x i32> %tmp5, <4 x i32> undef, <4 x i32> <i32 1, i32 undef, i32

Hi Johannes, >> Not sure the semantics of "used" you propose is sufficient. AFAIU the >> proposal, "used" could only be used in cases where the function will >> always trigger UB if poison is passed as argument. The semantics of >> attributes is usually the other way around, since function calls need >> to have UB as strong as the worst

You are right: select in SDAG has to be poison-blocking as well, otherwise the current lowering from IR's select to SDAG's select would be wrong. Which makes the select->or transformation incorrect at SDAG level as well. I guess until recently people believed that poison in SDAG wasn't much of a problem (myself included). I was convinced otherwise with the test cases that

On 02/19, Juneyoung Lee via llvm-dev wrote: > Hello, > > > Would it be correct to resolve this by saying that dereferenceable(N) > > *implies* not_poison? This would be helpful as a clarification of how > > it all fits together. > > Yes, I think it makes sense. I don't we should do that. Take the `gep inbounds` example: char* foo(char *arg) { return `gep

> -----Original Message----- > From: Chris Lattner <clattner at nondot.org> > Sent: Wednesday, February 5, 2020 4:02 PM > To: Eli Friedman <efriedma at quicinc.com> > Cc: llvm-dev <llvm-dev at lists.llvm.org> > Subject: [EXT] Re: [llvm-dev] [RFC] Extending shufflevector for vscale vectors > (SVE etc.) > > On Jan 29, 2020, at 4:48 PM, Eli Friedman via

Two thoughts: 1. I think that we should aim for regularity, to the extent possible, and so we should treat nonnull, align, etc. similarly w.r.t. to whether they produce poison or UB. 2. I was thinking about the following last night, and it clarified for me why having an not_poison attribute makes sense and seems useful, and how poison/UB might affect things on a function-call boundary itself.

Right - I think that case looks like this for little endian: define <2 x i32> @zextshuffle(<2 x i16> %x) { %zext_shuffle = shufflevector <2 x i16> %x, <2 x i16> zeroinitializer, <4 x i32> <i32 0, i32 2, i32 1, i32 2> %bc = bitcast <4 x i16> %zext_shuffle to <2 x i32> ret <2 x i32> %bc } define <2 x i32> @zextvec(<2 x i16>

We use InstCombiner::ShouldChangeType() to prevent transforms to illegal integer types, but I'm not sure how that would apply to vector types. Ie, let's say v256 is a legal type in your example. DataLayout doesn't appear to specify what configurations of a 256-bit vector are legal, so I don't think we can currently use that to say v2i128 should be treated differently than v16i16.

> -----Original Message----- > From: Chris Lattner <clattner at nondot.org> > Sent: Friday, February 7, 2020 3:00 PM > To: Eli Friedman <efriedma at quicinc.com> > Cc: llvm-dev <llvm-dev at lists.llvm.org> > Subject: [EXT] Re: [llvm-dev] [RFC] Extending shufflevector for vscale vectors > (SVE etc.) > > > On Feb 7, 2020, at 12:39 PM, Eli Friedman

Hi, Over the past few years we've been trying to kill poison somehow. There have been a few proposals, but they've all failed to pass the bar and/or to gather significant support (my own proposals included). We (David, Gil, John, Juneyoung, Sanjoy, Youngju, Yoonseung, and myself) have a new proposal to kill undef instead and replace it with poison + a new 'freeze' instruction. We

As Sanjay noted in D31426<https://reviews.llvm.org/D31426#712701>, InstructionSimplify is missing the following simplification: This function: define <4 x i32> @splat_operand(<4 x i32> %x) { %splat = shufflevector <4 x i32> %x, <4 x i32> undef, <4 x i32> zeroinitializer %shuf = shufflevector <4 x i32> %splat, <4 x i32> undef, <4 x i32>

On Thu, Jan 19, 2017 at 9:17 AM, Rackover, Zvi <zvi.rackover at intel.com> wrote: > Hi Sanjay, > > > > I agree we should also discuss **if** this canonicalization is beneficial. > > For starters, do we have a concrete case where we would benefit from > canonicalizing shuffles <-> truncates in LLVM IR? > > IMO, we should not count benefits for codegen

Not sure the semantics of "used" you propose is sufficient. AFAIU the proposal, "used" could only be used in cases where the function will always trigger UB if poison is passed as argument. The semantics of attributes is usually the other way around, since function calls need to have UB as strong as the worst behavior of the function. If a function may for some reason trigger

Cool, thanks for debugging this issue and letting us know. We have a few patches to fix this issue: - Introduce freeze in IR: https://reviews.llvm.org/D29011 - Lowering freeze: https://reviews.llvm.org/D29014 - Fix loop unswitch: https://reviews.llvm.org/D29015 Bonus patches to recover perf: - Be less conservative in loop unswitching: https://reviews.llvm.org/D29016 - Instcombine support