[AMDGPU] truncate left shift

[rampitec]

Summary

That is pretty common for clang to produce code like
(shl %x, (and %amt, 31)). In this situation we can still perform
trunc (shl) into shl (trunc) conversion given the known value
range of shift amount.

[arsenm]

lib/Target/AMDGPU/AMDGPUISelLowering.cpp

2761–2762 ↗

(On Diff #104306)

This exact combine is already done in DAGCombiner:

// trunc (shl x, K) -> shl (trunc x), K => K < VT.getScalarSizeInBits()

Why isn't it triggering? Is the other combine you added missing an AddToWorklist or something?

[rampitec]

lib/Target/AMDGPU/AMDGPUISelLowering.cpp

2761–2762 ↗

(On Diff #104306)

Ah. That one only works with constants.

[rampitec]

lib/Target/AMDGPU/AMDGPUISelLowering.cpp

2761–2762 ↗

(On Diff #104306)

Do you want me to transfer it there?

[arsenm]

lib/Target/AMDGPU/AMDGPUISelLowering.cpp

2761–2762 ↗

(On Diff #104306)

Yes, it would be better if it was all in one place

[rampitec]

Moved implementation into DAGCombiner and replaced existing DAGCombiner's optimization which was only capable of dealing with constant shift amounts.

[arsenm]

lib/CodeGen/SelectionDAG/DAGCombiner.cpp

8215

Will this regress the constant vector case?

[rampitec]

lib/CodeGen/SelectionDAG/DAGCombiner.cpp

8215

No.

[arsenm]

LGTM

[rampitec]

Added vector by constant vector testcase.

[arsenm]

The vector case won't matter for us, but maybe for other targets with legal vector operations

lib/CodeGen/SelectionDAG/DAGCombiner.cpp

if ((!LegalOperations || TLI.isOperationLegal(ISD::SELECT, SrcVT)) &&
       return DAG.getNode(ISD::SELECT, SDLoc(N), VT, Cond, TruncOp0, TruncOp1);
     }
   }
 
   // trunc (shl x, K) -> shl (trunc x), K => K < VT.getScalarSizeInBits()
   if (N0.getOpcode() == ISD::SHL && N0.hasOneUse() &&
       (!LegalOperations || TLI.isOperationLegalOrCustom(ISD::SHL, VT)) &&
       TLI.isTypeDesirableForOp(ISD::SHL, VT)) {
-    if (const ConstantSDNode *CAmt = isConstOrConstSplat(N0.getOperand(1))) {
-      uint64_t Amt = CAmt->getZExtValue();
-      unsigned Size = VT.getScalarSizeInBits();
-
-      if (Amt < Size) {
-        SDLoc SL(N);
-        EVT AmtVT = TLI.getShiftAmountTy(VT, DAG.getDataLayout());
+    SDValue Amt = N0.getOperand(1);
+    KnownBits Known;
+    DAG.computeKnownBits(Amt, Known);
+    if (Known.getBitWidth() - Known.countMinLeadingZeros() <= Log2_32(Size)) {
 
-        SDValue Trunc = DAG.getNode(ISD::TRUNCATE, SL, VT, N0.getOperand(0));
-        return DAG.getNode(ISD::SHL, SL, VT, Trunc,
-                           DAG.getConstant(Amt, SL, AmtVT));
+      if (AmtVT != Amt.getValueType()) {
+        Amt = DAG.getZExtOrTrunc(Amt, SL, AmtVT);
+        AddToWorklist(Amt.getNode());
       }
+      return DAG.getNode(ISD::SHL, SL, VT, Trunc, Amt);
     }
   }
 
   // Fold a series of buildvector, bitcast, and truncate if possible.
   // For example fold
   //   (2xi32 trunc (bitcast ((4xi32)buildvector x, x, y, y) 2xi64)) to
   //   (2xi32 (buildvector x, y)).
   if (Level == AfterLegalizeVectorOps && VT.isVector() &&

test/CodeGen/AMDGPU/alignbit-pat.ll

bb:
   %tmp3 = and i32 %arg2, 31
   %tmp4 = zext i32 %tmp3 to i64
   %tmp5 = lshr i64 %tmp, %tmp4
   %tmp6 = trunc i64 %tmp5 to i32
   store i32 %tmp6, i32 addrspace(1)* %arg1, align 4
   ret void
 }
 
-; GCN-LABEL: {{^}}alignbit_shl_pat:
-; GCN-DAG: s_load_dword s[[SHL:[0-9]+]]
-; GCN-DAG: load_dwordx2 v{{\[}}[[LO:[0-9]+]]:[[HI:[0-9]+]]{{\]}}
-; GCN-DAG: s_sub_i32 s[[SHR:[0-9]+]], 32, s[[SHL]]
-; GCN:     v_alignbit_b32 v{{[0-9]+}}, v[[HI]], v[[LO]], s[[SHR]]
-
-define amdgpu_kernel void @alignbit_shl_pat(i64 addrspace(1)* nocapture readonly %arg, i32 addrspace(1)* nocapture %arg1, i32 %arg2) {
-bb:
-  %tmp = load i64, i64 addrspace(1)* %arg, align 8
-  %tmp3 = and i32 %arg2, 31
-  %tmp4 = zext i32 %tmp3 to i64
-  %tmp5 = shl i64 %tmp, %tmp4
-  %tmp6 = trunc i64 %tmp5 to i32
-  store i32 %tmp6, i32 addrspace(1)* %arg1, align 4
-  ret void
-}
-
 ; GCN-LABEL: {{^}}alignbit_shr_pat_v:
 ; GCN-DAG: load_dword v[[SHR:[0-9]+]],
 ; GCN-DAG: load_dwordx2 v{{\[}}[[LO:[0-9]+]]:[[HI:[0-9]+]]{{\]}}
 ; GCN: v_alignbit_b32 v{{[0-9]+}}, v[[HI]], v[[LO]], v[[SHR]]
 
 define amdgpu_kernel void @alignbit_shr_pat_v(i64 addrspace(1)* nocapture readonly %arg, i32 addrspace(1)* nocapture %arg1) {
 bb:
   %tid = tail call i32 @llvm.amdgcn.workitem.id.x()
   %gep1 = getelementptr inbounds i64, i64 addrspace(1)* %arg, i32 %tid
   %tmp = load i64, i64 addrspace(1)* %gep1, align 8
   %gep2 = getelementptr inbounds i32, i32 addrspace(1)* %arg1, i32 %tid
   %amt = load i32, i32 addrspace(1)* %gep2, align 4
   %tmp3 = and i32 %amt, 31
   %tmp4 = zext i32 %tmp3 to i64
   %tmp5 = lshr i64 %tmp, %tmp4
   %tmp6 = trunc i64 %tmp5 to i32
   store i32 %tmp6, i32 addrspace(1)* %gep2, align 4
   ret void
 }
 
-; GCN-LABEL: {{^}}alignbit_shl_pat_v:
-; GCN-DAG: load_dword v[[SHL:[0-9]+]],
-; GCN-DAG: load_dwordx2 v{{\[}}[[LO:[0-9]+]]:[[HI:[0-9]+]]{{\]}}
-; GCN-DAG: v_sub_i32_e32 v[[SHR:[0-9]+]], {{[^,]+}}, 32, v[[SHL]]
-; GCN: v_alignbit_b32 v{{[0-9]+}}, v[[HI]], v[[LO]], v[[SHR]]
-
-define amdgpu_kernel void @alignbit_shl_pat_v(i64 addrspace(1)* nocapture readonly %arg, i32 addrspace(1)* nocapture %arg1) {
-bb:
-  %tid = tail call i32 @llvm.amdgcn.workitem.id.x()
-  %gep1 = getelementptr inbounds i64, i64 addrspace(1)* %arg, i32 %tid
-  %tmp = load i64, i64 addrspace(1)* %gep1, align 8
-  %gep2 = getelementptr inbounds i32, i32 addrspace(1)* %arg1, i32 %tid
-  %amt = load i32, i32 addrspace(1)* %gep2, align 4
-  %tmp3 = and i32 %amt, 31
-  %tmp4 = zext i32 %tmp3 to i64
-  %tmp5 = shl i64 %tmp, %tmp4
-  %tmp6 = trunc i64 %tmp5 to i32
-  store i32 %tmp6, i32 addrspace(1)* %gep2, align 4
-  ret void
-}
-
 ; GCN-LABEL: {{^}}alignbit_shr_pat_wrong_and30:
 ; Negative test, wrong constant
 ; GCN: v_lshr_b64
 ; GCN-NOT: v_alignbit_b32
 
 define amdgpu_kernel void @alignbit_shr_pat_wrong_and30(i64 addrspace(1)* nocapture readonly %arg, i32 addrspace(1)* nocapture %arg1, i32 %arg2) {
 bb:
   %tmp = load i64, i64 addrspace(1)* %arg, align 8
   %tmp3 = and i32 %arg2, 30
   %tmp4 = zext i32 %tmp3 to i64
   %tmp5 = lshr i64 %tmp, %tmp4
   %tmp6 = trunc i64 %tmp5 to i32
   store i32 %tmp6, i32 addrspace(1)* %arg1, align 4
   ret void
 }
 
-; GCN-LABEL: {{^}}alignbit_shl_pat_wrong_and30:
-; Negative test, wrong constant
-; GCN: v_lshl_b64
-; GCN-NOT: v_alignbit_b32
-
-define amdgpu_kernel void @alignbit_shl_pat_wrong_and30(i64 addrspace(1)* nocapture readonly %arg, i32 addrspace(1)* nocapture %arg1, i32 %arg2) {
-bb:
-  %tmp = load i64, i64 addrspace(1)* %arg, align 8
-  %tmp3 = and i32 %arg2, 30
-  %tmp4 = zext i32 %tmp3 to i64
-  %tmp5 = shl i64 %tmp, %tmp4
-  %tmp6 = trunc i64 %tmp5 to i32
-  store i32 %tmp6, i32 addrspace(1)* %arg1, align 4
-  ret void
-}
-
 ; GCN-LABEL: {{^}}alignbit_shr_pat_wrong_and63:
 ; Negative test, wrong constant
 ; GCN: v_lshr_b64
 ; GCN-NOT: v_alignbit_b32
 
 define amdgpu_kernel void @alignbit_shr_pat_wrong_and63(i64 addrspace(1)* nocapture readonly %arg, i32 addrspace(1)* nocapture %arg1, i32 %arg2) {
 bb:
   %tmp = load i64, i64 addrspace(1)* %arg, align 8
   %tmp3 = and i32 %arg2, 63
   %tmp4 = zext i32 %tmp3 to i64
   %tmp5 = lshr i64 %tmp, %tmp4
   %tmp6 = trunc i64 %tmp5 to i32
   store i32 %tmp6, i32 addrspace(1)* %arg1, align 4
   ret void
 }
 
-; GCN-LABEL: {{^}}alignbit_shl_pat_wrong_and63:
-; Negative test, wrong constant
-; GCN: v_lshl_b64
-; GCN-NOT: v_alignbit_b32
-
-define amdgpu_kernel void @alignbit_shl_pat_wrong_and63(i64 addrspace(1)* nocapture readonly %arg, i32 addrspace(1)* nocapture %arg1, i32 %arg2) {
-bb:
-  %tmp = load i64, i64 addrspace(1)* %arg, align 8
-  %tmp3 = and i32 %arg2, 63
-  %tmp4 = zext i32 %tmp3 to i64
-  %tmp5 = shl i64 %tmp, %tmp4
-  %tmp6 = trunc i64 %tmp5 to i32
-  store i32 %tmp6, i32 addrspace(1)* %arg1, align 4
-  ret void
-}
 declare i32 @llvm.amdgcn.workitem.id.x() #0
 
 attributes #0 = { nounwind readnone speculatable }

test/CodeGen/AMDGPU/shift-i64-opts.ll

 define amdgpu_kernel void @trunc_shl_31_i32_i64_multi_use(i32 addrspace(1)* %out, i64 addrspace(1)* %in) {
   %val = load i64, i64 addrspace(1)* %in
   %shl = shl i64 %val, 31
   %trunc = trunc i64 %shl to i32
   store volatile i32 %trunc, i32 addrspace(1)* %out
   store volatile i64 %shl, i64 addrspace(1)* %in
   ret void
 }
+
+; GCN-LABEL: {{^}}trunc_shl_and31:
+; GCN:     s_and_b32 s[[AMT:[0-9]+]], s{{[0-9]+}}, 31
+; GCN:     v_lshlrev_b32_e32 v{{[0-9]+}}, s[[AMT]], v{{[0-9]+}}
+; GCN-NOT: v_lshl_b64
+; GCN-NOT: v_lshlrev_b64
+define amdgpu_kernel void @trunc_shl_and31(i64 addrspace(1)* nocapture readonly %arg, i32 addrspace(1)* nocapture %arg1, i32 %arg2) {
+bb:
+  %tmp = load i64, i64 addrspace(1)* %arg, align 8
+  %tmp3 = and i32 %arg2, 31
+  %tmp4 = zext i32 %tmp3 to i64
+  %tmp5 = shl i64 %tmp, %tmp4
+  %tmp6 = trunc i64 %tmp5 to i32
+  store i32 %tmp6, i32 addrspace(1)* %arg1, align 4
+  ret void
+}
+
+; GCN-LABEL: {{^}}trunc_shl_and30:
+; GCN:     s_and_b32 s[[AMT:[0-9]+]], s{{[0-9]+}}, 30
+; GCN:     v_lshlrev_b32_e32 v{{[0-9]+}}, s[[AMT]], v{{[0-9]+}}
+; GCN-NOT: v_lshl_b64
+; GCN-NOT: v_lshlrev_b64
+define amdgpu_kernel void @trunc_shl_and30(i64 addrspace(1)* nocapture readonly %arg, i32 addrspace(1)* nocapture %arg1, i32 %arg2) {
+bb:
+  %tmp = load i64, i64 addrspace(1)* %arg, align 8
+  %tmp3 = and i32 %arg2, 30
+  %tmp4 = zext i32 %tmp3 to i64
+  %tmp5 = shl i64 %tmp, %tmp4
+  %tmp6 = trunc i64 %tmp5 to i32
+  store i32 %tmp6, i32 addrspace(1)* %arg1, align 4
+  ret void
+}
+
+; GCN-LABEL: {{^}}trunc_shl_wrong_and63:
+; Negative test, wrong constant
+; GCN: v_lshl_b64
+define amdgpu_kernel void @trunc_shl_wrong_and63(i64 addrspace(1)* nocapture readonly %arg, i32 addrspace(1)* nocapture %arg1, i32 %arg2) {
+bb:
+  %tmp = load i64, i64 addrspace(1)* %arg, align 8
+  %tmp3 = and i32 %arg2, 63
+  %tmp4 = zext i32 %tmp3 to i64
+  %tmp5 = shl i64 %tmp, %tmp4
+  %tmp6 = trunc i64 %tmp5 to i32
+  store i32 %tmp6, i32 addrspace(1)* %arg1, align 4
+  ret void
+}
+
+; GCN-LABEL: {{^}}trunc_shl_no_and:
+; Negative test, shift can be full 64 bit
+; GCN: v_lshl_b64
+define amdgpu_kernel void @trunc_shl_no_and(i64 addrspace(1)* nocapture readonly %arg, i32 addrspace(1)* nocapture %arg1, i32 %arg2) {
+bb:
+  %tmp = load i64, i64 addrspace(1)* %arg, align 8
+  %tmp4 = zext i32 %arg2 to i64
+  %tmp5 = shl i64 %tmp, %tmp4
+  %tmp6 = trunc i64 %tmp5 to i32
+  store i32 %tmp6, i32 addrspace(1)* %arg1, align 4
+  ret void
+}