loop-vectorize miscompile #142957
Description
llvm commit: 6955a7d
Reproduce with:
opt -passes="loop-vectorize" bbi-107525_lv.ll -S -o - -mtriple=aarch64-none-linux-gnu --data-layout="p:16:16"
The loop-vectorize output contains
pred.sdiv.continue: ; preds = %pred.sdiv.if, %vector.body
%3 = phi i16 [ poison, %vector.body ], [ %2, %pred.sdiv.if ]
[...]
pred.sdiv.continue2: ; preds = %pred.sdiv.if1, %pred.sdiv.continue
[...]
%7 = shl i16 %3, 14
%8 = sub i16 %6, %7
%9 = getelementptr [16 x i64], ptr @g_855, i16 0, i16 %8
%10 = getelementptr i64, ptr %9, i32 0
%wide.load = load <2 x i64>, ptr %10, align 1
and the problem here is that we will get to the %3 phi from %vector.body and then continue to pred.sdiv.continue2 where the %3 poison value will make %7, %8, %9 and %10 poison, and then we will do a load from %10 which is instant UB.
The input program has no UB.
Before 0d61ffd
[Loads] Support SCEVAddExpr as start for pointer AddRec.
the result is ok. Then with 0d61ffd it crashes with
opt: ../include/llvm/Support/Casting.h:109: static bool llvm::isa_impl_cl<llvm::Instruction, const llvm::Value *>::doit(const From *) [To = llvm::Instruction, From = const llvm::Value *]: Assertion `Val && "isa<> used on a null pointer"' failed.
and that crash goes away with fd82b5b
[LV] Support recieps without underlying instr in collectPoisonGenRec.
Support recipes without underlying instruction in
collectPoisonGeneratingRecipes by directly trying to dyn_cast_or_null
the underlying value.
Fixes https://github.com/llvm/llvm-project/issues/70590.
but then we get the miscompile instead.
I also tested to cherry-pick the fd82b5b to directly after 0d61ffd and we get the miscompile then too, so it doesn't seem to be some other patch between them that causes the miscompile.