[SLP] Don't allow Div/Rem as alternate opcodes

Summary:
We don't have control/verify what will be the RHS of the division, so it might
happen to be zero, causing UB.

Reviewers: Vasilis, RKSimon, ABataev

Reviewed By: ABataev

Subscribers: vporpo, ABataev, hiraditya, llvm-commits, vdmitrie

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D72740

(cherry picked from commit e1d6d368529322edc658c893c01eaadaf8053ea6)

1 files changed, 17 insertions, 1 deletions

diff --git a/llvm/lib/Transforms/Vectorize/SLPVectorizer.cpp b/llvm/lib/Transforms/Vectorize/SLPVectorizer.cpp
index aabd974cd73..479bca83b51 100644
--- a/llvm/lib/Transforms/Vectorize/SLPVectorizer.cpp
+++ b/llvm/lib/Transforms/Vectorize/SLPVectorizer.cpp
@@ -377,6 +377,18 @@ static Value *isOneOf(const InstructionsState &S, Value *Op) {
   return S.OpValue;
 }
 
+/// \returns true if \p Opcode is allowed as part of of the main/alternate
+/// instruction for SLP vectorization.
+///
+/// Example of unsupported opcode is SDIV that can potentially cause UB if the
+/// "shuffled out" lane would result in division by zero.
+static bool isValidForAlternation(unsigned Opcode) {
+  if (Instruction::isIntDivRem(Opcode))
+    return false;
+
+  return true;
+}
+
 /// \returns analysis of the Instructions in \p VL described in
 /// InstructionsState, the Opcode that we suppose the whole list
 /// could be vectorized even if its structure is diverse.
@@ -399,7 +411,8 @@ static InstructionsState getSameOpcode(ArrayRef<Value *> VL,
     if (IsBinOp && isa<BinaryOperator>(VL[Cnt])) {
       if (InstOpcode == Opcode || InstOpcode == AltOpcode)
         continue;
-      if (Opcode == AltOpcode) {
+      if (Opcode == AltOpcode && isValidForAlternation(InstOpcode) &&
+          isValidForAlternation(Opcode)) {
         AltOpcode = InstOpcode;
         AltIndex = Cnt;
         continue;
@@ -411,6 +424,9 @@ static InstructionsState getSameOpcode(ArrayRef<Value *> VL,
         if (InstOpcode == Opcode || InstOpcode == AltOpcode)
           continue;
         if (Opcode == AltOpcode) {
+          assert(isValidForAlternation(Opcode) &&
+                 isValidForAlternation(InstOpcode) &&
+                 "Cast isn't safe for alternation, logic needs to be updated!");
           AltOpcode = InstOpcode;
           AltIndex = Cnt;
           continue;