[SDAG] Transfer gep nusw/nuw to SDAG

The resulting add is nuw if either the gep was nuw or it was
nusw+nneg. Previously only inbounds+nneg was handled.

Test via wasm load offsets, which seems to most directly expose
these SDAG flags.

Author: nikic

llvm/lib/CodeGen/SelectionDAG/SelectionDAGBuilder.cpp

@@ -4287,6 +4287,7 @@ void SelectionDAGBuilder::visitGetElementPtr(const User &I) {
   SDValue N = getValue(Op0);
   SDLoc dl = getCurSDLoc();
   auto &TLI = DAG.getTargetLoweringInfo();
+  GEPNoWrapFlags NW = cast<GEPOperator>(I).getNoWrapFlags();
 
   // Normalize Vector GEP - all scalar operands should be converted to the
   // splat vector.
@@ -4314,7 +4315,8 @@ void SelectionDAGBuilder::visitGetElementPtr(const User &I) {
         // In an inbounds GEP with an offset that is nonnegative even when
         // interpreted as signed, assume there is no unsigned overflow.
         SDNodeFlags Flags;
-        if (int64_t(Offset) >= 0 && cast<GEPOperator>(I).isInBounds())
+        if (NW.hasNoUnsignedWrap() ||
+            (int64_t(Offset) >= 0 && NW.hasNoUnsignedSignedWrap()))
           Flags.setNoUnsignedWrap(true);
 
         N = DAG.getNode(ISD::ADD, dl, N.getValueType(), N,
@@ -4355,7 +4357,8 @@ void SelectionDAGBuilder::visitGetElementPtr(const User &I) {
         // In an inbounds GEP with an offset that is nonnegative even when
         // interpreted as signed, assume there is no unsigned overflow.
         SDNodeFlags Flags;
-        if (Offs.isNonNegative() && cast<GEPOperator>(I).isInBounds())
+        if (NW.hasNoUnsignedWrap() ||
+            (Offs.isNonNegative() && NW.hasNoUnsignedSignedWrap()))
           Flags.setNoUnsignedWrap(true);
 
         OffsVal = DAG.getSExtOrTrunc(OffsVal, dl, N.getValueType());

llvm/test/CodeGen/WebAssembly/offset.ll

@@ -40,6 +40,26 @@ define i32 @load_i32_with_folded_gep_offset(ptr %p) {
   ret i32 %t
 }
 
+; Same for nusw.
+
+; CHECK-LABEL: load_i32_with_folded_gep_offset_nusw:
+; CHECK: i32.load  $push0=, 24($0){{$}}
+define i32 @load_i32_with_folded_gep_offset_nusw(ptr %p) {
+  %s = getelementptr nusw i32, ptr %p, i32 6
+  %t = load i32, ptr %s
+  ret i32 %t
+}
+
+; For nuw we don't need the offset to be positive.
+
+; CHECK-LABEL: load_i32_with_folded_gep_offset_nuw:
+; CHECK: i32.load  $push0=, -24($0){{$}}
+define i32 @load_i32_with_folded_gep_offset_nuw(ptr %p) {
+  %s = getelementptr nuw i32, ptr %p, i32 -6
+  %t = load i32, ptr %s
+  ret i32 %t
+}
+
 ; We can't fold a negative offset though, even with an inbounds gep.
 
 ; CHECK-LABEL: load_i32_with_unfolded_gep_negative_offset: