[DAG] getNode() - fold (zext (trunc x)) -> x iff the upper bits are k…

…nown zero - add SRL support

This is part of the work to address the D155472 regressions, there's a number of issues with generalizing this fold which is why I'm just adding SRL support atm.

Differential Revision: https://reviews.llvm.org/D159533

llvm/lib/CodeGen/SelectionDAG/SelectionDAG.cpp

@@ -5695,12 +5695,13 @@ SDValue SelectionDAG::getNode(unsigned Opcode, const SDLoc &DL, EVT VT,
       return getConstant(0, DL, VT);
 
     // Skip unnecessary zext_inreg pattern:
-    // (zext (trunc (assertzext x))) -> (assertzext x)
-    // TODO: Generalize to MaskedValueIsZero check?
+    // (zext (trunc x)) -> x iff the upper bits are known zero.
+    // TODO: Generalize to just the MaskedValueIsZero check?
     if (OpOpcode == ISD::TRUNCATE) {
       SDValue OpOp = N1.getOperand(0);
       if (OpOp.getValueType() == VT) {
-        if (OpOp.getOpcode() == ISD::AssertZext) {
+        if (OpOp.getOpcode() == ISD::AssertZext ||
+            OpOp.getOpcode() == ISD::SRL) {
           APInt HiBits = APInt::getBitsSetFrom(VT.getScalarSizeInBits(),
                                                N1.getScalarValueSizeInBits());
           if (MaskedValueIsZero(OpOp, HiBits)) {

llvm/test/CodeGen/AMDGPU/ctpop16.ll

@@ -1523,9 +1523,8 @@ define amdgpu_kernel void @ctpop_i16_in_br(ptr addrspace(1) %out, ptr addrspace(
 ; VI-NEXT:    s_load_dwordx4 s[0:3], s[0:1], 0x24
 ; VI-NEXT:    s_waitcnt lgkmcnt(0)
 ; VI-NEXT:    s_lshr_b32 s5, s4, 16
-; VI-NEXT:    v_cmp_ne_u16_e64 s[6:7], s5, 0
-; VI-NEXT:    s_and_b64 vcc, exec, s[6:7]
-; VI-NEXT:    s_cbranch_vccz .LBB14_4
+; VI-NEXT:    s_cmp_lg_u32 s5, 0
+; VI-NEXT:    s_cbranch_scc0 .LBB14_4
 ; VI-NEXT:  ; %bb.1: ; %else
 ; VI-NEXT:    s_mov_b32 s11, 0xf000
 ; VI-NEXT:    s_mov_b32 s10, -1

llvm/test/CodeGen/AMDGPU/permute_i8.ll

@@ -353,17 +353,16 @@ define hidden void @shuffle5341ud2(ptr addrspace(1) %in0, ptr addrspace(1) %out0
 ; GFX10-NEXT:    s_waitcnt vmcnt(0) expcnt(0) lgkmcnt(0)
 ; GFX10-NEXT:    global_load_dword v0, v[0:1], off
 ; GFX10-NEXT:    s_waitcnt vmcnt(0)
-; GFX10-NEXT:    v_perm_b32 v0, v0, v0, 0x5040706
+; GFX10-NEXT:    v_alignbit_b32 v0, v0, v0, 16
 ; GFX10-NEXT:    global_store_dword v[2:3], v0, off
 ; GFX10-NEXT:    s_setpc_b64 s[30:31]
 ;
 ; GFX9-LABEL: shuffle5341ud2:
 ; GFX9:       ; %bb.0:
 ; GFX9-NEXT:    s_waitcnt vmcnt(0) expcnt(0) lgkmcnt(0)
 ; GFX9-NEXT:    global_load_dword v0, v[0:1], off
-; GFX9-NEXT:    s_mov_b32 s4, 0x5040706
 ; GFX9-NEXT:    s_waitcnt vmcnt(0)
-; GFX9-NEXT:    v_perm_b32 v0, v0, v0, s4
+; GFX9-NEXT:    v_alignbit_b32 v0, v0, v0, 16
 ; GFX9-NEXT:    global_store_dword v[2:3], v0, off
 ; GFX9-NEXT:    s_waitcnt vmcnt(0)
 ; GFX9-NEXT:    s_setpc_b64 s[30:31]