Addresses nit, added negative test case

Author: badumbatish

llvm/lib/Analysis/ConstantFolding.cpp

@@ -3831,8 +3831,8 @@ static Constant *ConstantFoldFixedVectorCall(
     unsigned NumElements =
         cast<FixedVectorType>(Operands[0]->getType())->getNumElements();
 
-    assert(NumElements == 8 && NumElements / 2 == Result.size() &&
-           "wasm dot takes i16x8 and produce i32x4");
+    assert(NumElements == 8 && Result.size() == 4 &&
+           "wasm dot takes i16x8 and produces i32x4");
     assert(Ty->isIntegerTy());
     SmallVector<APInt, 8> MulVector;
 
@@ -3845,11 +3845,13 @@ static Constant *ConstantFoldFixedVectorCall(
       // sext 32 first, according to specs
       APInt IMul = Elt0->getValue().sext(32) * Elt1->getValue().sext(32);
 
-      // i16 -> i32 bypasses specs modulo on imul
+      // Multiplication can never be more than 32 bit.
+      // We can opt to not perform modulo of imul here.
       MulVector.push_back(IMul);
     }
     for (unsigned I = 0; I < Result.size(); I++) {
-      // i16 -> i32 bypasses specs modulo on iadd
+      // Addition can never be more than 32 bit.
+      // We can opt to not perform modulo of iadd here.
       APInt IAdd = MulVector[I * 2] + MulVector[I * 2 + 1];
       Result[I] = ConstantInt::get(Ty, IAdd);
     }

llvm/test/Transforms/InstSimplify/ConstProp/WebAssembly/dot.ll

@@ -41,11 +41,22 @@ define <4 x i32> @dot_doubly_negative() {
 ;   subsequent modulo of 2^32 of imul and iadd
 ;   should return the same result
 ; 2*(2^15 - 1)^2 % 2^32 == 2*(2^15 - 1)^2
-define <4 x i32> @dot_follow_modulo_spec() {
-; CHECK-LABEL: define <4 x i32> @dot_follow_modulo_spec() {
+define <4 x i32> @dot_follow_modulo_spec_1() {
+; CHECK-LABEL: define <4 x i32> @dot_follow_modulo_spec_1() {
 ; CHECK-NEXT:    ret <4 x i32> <i32 2147352578, i32 0, i32 0, i32 0>
 ;
   %res = tail call <4 x i32> @llvm.wasm.dot(<8 x i16> <i16 32767, i16 32767, i16 0, i16 0, i16 0, i16 0, i16 0, i16 0>, <8 x i16> <i16 32767, i16 32767, i16 0, i16 0, i16 0, i16 0, i16 0, i16 0>)
   ret <4 x i32> %res
 }
 
+; This test checks for llvm's compliance on spec's wasm.dot's imul and iadd
+; 2*(- 2^15)^2 == 2^31, doesn't exceed 2^32 so we don't have to mod
+; wrapping around is -(2^31), still doesn't exceed 2^32
+define <4 x i32> @dot_follow_modulo_spec_2() {
+; CHECK-LABEL: define <4 x i32> @dot_follow_modulo_spec_2() {
+; CHECK-NEXT:    ret <4 x i32> <i32 -2147483648, i32 0, i32 0, i32 0>
+;
+  %res = tail call <4 x i32> @llvm.wasm.dot(<8 x i16> <i16 -32768, i16 -32768, i16 0, i16 0, i16 0, i16 0, i16 0, i16 0>, <8 x i16> <i16 -32768, i16 -32768, i16 0, i16 0, i16 0, i16 0, i16 0, i16 0>)
+  ret <4 x i32> %res
+}
+