[GlobalISel] ComputeKnownBits - use common KnownBits shift handling (PR44526)

[RKSimon]

Summary

Convert GISelKnownBits.computeKnownBitsImpl shift handling to use the common KnownBits implementations, which makes use the of the known leading/trailing bits for shifted values in cases where we don't know the shift amount value as detailed in https://blog.regehr.org/archives/1709 .

[arsenm]

llvm/lib/CodeGen/GlobalISel/GISelKnownBits.cpp

372–375

I thought there was a move towards moving the shared implementation of these into KnownBits::. Is there a reason this needs to be repeated here?

379

s/signbits/sign bits/

[RKSimon]

llvm/lib/CodeGen/GlobalISel/GISelKnownBits.cpp

372–375

No reason - I'll refactor the SelectionDAG version to do that and then update this to use it as well.

[RKSimon]

Updated to use the common KnownBits implementations.

llvm/lib/CodeGen/GlobalISel/GISelKnownBits.cpp

case TargetOpcode::G_LOAD: {
 
     break;
   }
   case TargetOpcode::G_ZEXTLOAD: {
     // Everything above the retrieved bits is zero
     Known.Zero.setBitsFrom((*MI.memoperands_begin())->getSizeInBits());
     break;
   }
-  case TargetOpcode::G_ASHR:
-  case TargetOpcode::G_LSHR:
-  case TargetOpcode::G_SHL: {
-    KnownBits RHSKnown;
+  case TargetOpcode::G_ASHR: {
+    KnownBits LHSKnown, RHSKnown;
+    computeKnownBitsImpl(MI.getOperand(1).getReg(), LHSKnown, DemandedElts,
+                         Depth + 1);
     computeKnownBitsImpl(MI.getOperand(2).getReg(), RHSKnown, DemandedElts,
                          Depth + 1);
-    if (!RHSKnown.isConstant()) {
-      LLVM_DEBUG(
-          MachineInstr *RHSMI = MRI.getVRegDef(MI.getOperand(2).getReg());
-          dbgs() << '[' << Depth << "] Shift not known constant: " << *RHSMI);
-      break;
-    }
-    uint64_t Shift = RHSKnown.getConstant().getZExtValue();
-    LLVM_DEBUG(dbgs() << '[' << Depth << "] Shift is " << Shift << '\n');
-
-    // Guard against oversized shift amounts
-    if (Shift >= MRI.getType(MI.getOperand(1).getReg()).getScalarSizeInBits())
-      break;
-
-    computeKnownBitsImpl(MI.getOperand(1).getReg(), Known, DemandedElts,
+    Known = KnownBits::ashr(LHSKnown, RHSKnown);
+  case TargetOpcode::G_LSHR: {
+    KnownBits LHSKnown, RHSKnown;
+    computeKnownBitsImpl(MI.getOperand(1).getReg(), LHSKnown, DemandedElts,
                          Depth + 1);
-
-    switch (Opcode) {
-    case TargetOpcode::G_ASHR:
-      Known.Zero = Known.Zero.ashr(Shift);
-      Known.One = Known.One.ashr(Shift);
-      break;
-    case TargetOpcode::G_LSHR:
-      Known.Zero = Known.Zero.lshr(Shift);
-      Known.One = Known.One.lshr(Shift);
-      Known.Zero.setBitsFrom(Known.Zero.getBitWidth() - Shift);
-      break;
-    case TargetOpcode::G_SHL:
-      Known.Zero = Known.Zero.shl(Shift);
-      Known.One = Known.One.shl(Shift);
-      Known.Zero.setBits(0, Shift);
-      break;
-    }
+    computeKnownBitsImpl(MI.getOperand(2).getReg(), RHSKnown, DemandedElts,
+                         Depth + 1);
+    Known = KnownBits::lshr(LHSKnown, RHSKnown);
+  case TargetOpcode::G_SHL: {
+    KnownBits LHSKnown, RHSKnown;
+    computeKnownBitsImpl(MI.getOperand(1).getReg(), LHSKnown, DemandedElts,
+                         Depth + 1);
+    computeKnownBitsImpl(MI.getOperand(2).getReg(), RHSKnown, DemandedElts,
+                         Depth + 1);
+    Known = KnownBits::shl(LHSKnown, RHSKnown);
     break;
   }
   case TargetOpcode::G_INTTOPTR:
   case TargetOpcode::G_PTRTOINT:
     // Fall through and handle them the same as zext/trunc.
     LLVM_FALLTHROUGH;
   case TargetOpcode::G_ZEXT:
   case TargetOpcode::G_TRUNC: {

llvm/test/CodeGen/AMDGPU/GlobalISel/lshr.ll

 ; GFX6:       ; %bb.0:
 ; GFX6-NEXT:    s_mov_b32 s4, 0xffff
 ; GFX6-NEXT:    s_and_b32 s2, s2, s4
 ; GFX6-NEXT:    s_and_b32 s0, s0, s4
 ; GFX6-NEXT:    s_lshr_b32 s0, s0, s2
 ; GFX6-NEXT:    s_and_b32 s2, s3, s4
 ; GFX6-NEXT:    s_and_b32 s1, s1, s4
 ; GFX6-NEXT:    s_lshr_b32 s1, s1, s2
-; GFX6-NEXT:    s_and_b32 s1, s1, s4
-; GFX6-NEXT:    s_and_b32 s0, s0, s4
 ; GFX6-NEXT:    s_lshl_b32 s1, s1, 16
 ; GFX6-NEXT:    s_or_b32 s0, s0, s1
 ; GFX6-NEXT:    ; return to shader part epilog
 ;
 ; GFX8-LABEL: s_lshr_v2i16:
 ; GFX8:       ; %bb.0:
 ; GFX8-NEXT:    s_mov_b32 s3, 0xffff
 ; GFX8-NEXT:    s_lshr_b32 s2, s0, 16

 ; GFX6:       ; %bb.0:
 ; GFX6-NEXT:    s_mov_b32 s2, 0xffff
 ; GFX6-NEXT:    v_and_b32_e32 v0, s2, v0
 ; GFX6-NEXT:    s_and_b32 s0, s0, s2
 ; GFX6-NEXT:    v_lshr_b32_e32 v0, s0, v0
 ; GFX6-NEXT:    v_and_b32_e32 v1, s2, v1
 ; GFX6-NEXT:    s_and_b32 s0, s1, s2
 ; GFX6-NEXT:    v_lshr_b32_e32 v1, s0, v1
-; GFX6-NEXT:    v_and_b32_e32 v1, s2, v1
-; GFX6-NEXT:    v_and_b32_e32 v0, s2, v0
 ; GFX6-NEXT:    v_lshlrev_b32_e32 v1, 16, v1
 ; GFX6-NEXT:    v_or_b32_e32 v0, v0, v1
 ; GFX6-NEXT:    ; return to shader part epilog
 ;
 ; GFX8-LABEL: lshr_v2i16_sv:
 ; GFX8:       ; %bb.0:
 ; GFX8-NEXT:    s_lshr_b32 s1, s0, 16
 ; GFX8-NEXT:    v_mov_b32_e32 v2, s1

 ; GFX6:       ; %bb.0:
 ; GFX6-NEXT:    s_mov_b32 s2, 0xffff
 ; GFX6-NEXT:    s_and_b32 s0, s0, s2
 ; GFX6-NEXT:    v_and_b32_e32 v0, s2, v0
 ; GFX6-NEXT:    v_lshrrev_b32_e32 v0, s0, v0
 ; GFX6-NEXT:    s_and_b32 s0, s1, s2
 ; GFX6-NEXT:    v_and_b32_e32 v1, s2, v1
 ; GFX6-NEXT:    v_lshrrev_b32_e32 v1, s0, v1
-; GFX6-NEXT:    v_and_b32_e32 v1, s2, v1
-; GFX6-NEXT:    v_and_b32_e32 v0, s2, v0
 ; GFX6-NEXT:    v_lshlrev_b32_e32 v1, 16, v1
 ; GFX6-NEXT:    v_or_b32_e32 v0, v0, v1
 ; GFX6-NEXT:    ; return to shader part epilog
 ;
 ; GFX8-LABEL: lshr_v2i16_vs:
 ; GFX8:       ; %bb.0:
 ; GFX8-NEXT:    s_lshr_b32 s1, s0, 16
 ; GFX8-NEXT:    v_mov_b32_e32 v2, s1

 ; GFX6-NEXT:    v_and_b32_e32 v4, s4, v4
 ; GFX6-NEXT:    v_and_b32_e32 v0, s4, v0
 ; GFX6-NEXT:    v_lshrrev_b32_e32 v0, v4, v0
 ; GFX6-NEXT:    v_and_b32_e32 v4, s4, v5
 ; GFX6-NEXT:    v_and_b32_e32 v1, s4, v1
 ; GFX6-NEXT:    v_lshrrev_b32_e32 v1, v4, v1
 ; GFX6-NEXT:    v_and_b32_e32 v4, s4, v6
 ; GFX6-NEXT:    v_and_b32_e32 v2, s4, v2
-; GFX6-NEXT:    v_and_b32_e32 v1, s4, v1
 ; GFX6-NEXT:    v_lshrrev_b32_e32 v2, v4, v2
 ; GFX6-NEXT:    v_and_b32_e32 v4, s4, v7
 ; GFX6-NEXT:    v_and_b32_e32 v3, s4, v3
-; GFX6-NEXT:    v_lshrrev_b32_e32 v3, v4, v3
-; GFX6-NEXT:    v_and_b32_e32 v0, s4, v0
 ; GFX6-NEXT:    v_lshlrev_b32_e32 v1, 16, v1
+; GFX6-NEXT:    v_lshrrev_b32_e32 v3, v4, v3
 ; GFX6-NEXT:    v_or_b32_e32 v0, v0, v1
-; GFX6-NEXT:    v_and_b32_e32 v1, s4, v2
-; GFX6-NEXT:    v_and_b32_e32 v2, s4, v3
-; GFX6-NEXT:    v_lshlrev_b32_e32 v2, 16, v2
-; GFX6-NEXT:    v_or_b32_e32 v1, v1, v2
+; GFX6-NEXT:    v_lshlrev_b32_e32 v1, 16, v3
+; GFX6-NEXT:    v_or_b32_e32 v1, v2, v1
 ; GFX6-NEXT:    s_setpc_b64 s[30:31]
 ;
 ; GFX8-LABEL: v_lshr_v4i16:
 ; GFX8:       ; %bb.0:
 ; GFX8-NEXT:    s_waitcnt vmcnt(0) expcnt(0) lgkmcnt(0)
 ; GFX8-NEXT:    v_lshrrev_b16_e32 v4, v2, v0
 ; GFX8-NEXT:    v_lshrrev_b16_sdwa v0, v2, v0 dst_sel:WORD_1 dst_unused:UNUSED_PAD src0_sel:WORD_1 src1_sel:WORD_1
 ; GFX8-NEXT:    v_lshrrev_b16_e32 v2, v3, v1

 ; GFX6-NEXT:    s_and_b32 s4, s4, s8
 ; GFX6-NEXT:    s_and_b32 s0, s0, s8
 ; GFX6-NEXT:    s_lshr_b32 s0, s0, s4
 ; GFX6-NEXT:    s_and_b32 s4, s5, s8
 ; GFX6-NEXT:    s_and_b32 s1, s1, s8
 ; GFX6-NEXT:    s_lshr_b32 s1, s1, s4
 ; GFX6-NEXT:    s_and_b32 s4, s6, s8
 ; GFX6-NEXT:    s_and_b32 s2, s2, s8
-; GFX6-NEXT:    s_and_b32 s1, s1, s8
 ; GFX6-NEXT:    s_lshr_b32 s2, s2, s4
 ; GFX6-NEXT:    s_and_b32 s4, s7, s8
 ; GFX6-NEXT:    s_and_b32 s3, s3, s8
-; GFX6-NEXT:    s_lshr_b32 s3, s3, s4
-; GFX6-NEXT:    s_and_b32 s0, s0, s8
 ; GFX6-NEXT:    s_lshl_b32 s1, s1, 16
+; GFX6-NEXT:    s_lshr_b32 s3, s3, s4
 ; GFX6-NEXT:    s_or_b32 s0, s0, s1
-; GFX6-NEXT:    s_and_b32 s1, s2, s8
-; GFX6-NEXT:    s_and_b32 s2, s3, s8
-; GFX6-NEXT:    s_lshl_b32 s2, s2, 16
-; GFX6-NEXT:    s_or_b32 s1, s1, s2
+; GFX6-NEXT:    s_lshl_b32 s1, s3, 16
+; GFX6-NEXT:    s_or_b32 s1, s2, s1
 ; GFX6-NEXT:    ; return to shader part epilog
 ;
 ; GFX8-LABEL: s_lshr_v4i16:
 ; GFX8:       ; %bb.0:
 ; GFX8-NEXT:    s_mov_b32 s6, 0xffff
 ; GFX8-NEXT:    s_lshr_b32 s4, s0, 16
 ; GFX8-NEXT:    s_lshr_b32 s7, s2, 16
 ; GFX8-NEXT:    s_and_b32 s0, s0, s6

 ; GFX6-NEXT:    v_and_b32_e32 v8, s4, v9
 ; GFX6-NEXT:    v_and_b32_e32 v1, s4, v1
 ; GFX6-NEXT:    v_lshrrev_b32_e32 v1, v8, v1
 ; GFX6-NEXT:    v_and_b32_e32 v8, s4, v10
 ; GFX6-NEXT:    v_and_b32_e32 v2, s4, v2
 ; GFX6-NEXT:    v_lshrrev_b32_e32 v2, v8, v2
 ; GFX6-NEXT:    v_and_b32_e32 v8, s4, v11
 ; GFX6-NEXT:    v_and_b32_e32 v3, s4, v3
-; GFX6-NEXT:    v_mov_b32_e32 v16, 0xffff
 ; GFX6-NEXT:    v_lshrrev_b32_e32 v3, v8, v3
 ; GFX6-NEXT:    v_and_b32_e32 v8, s4, v12
 ; GFX6-NEXT:    v_and_b32_e32 v4, s4, v4
-; GFX6-NEXT:    v_and_b32_e32 v1, v1, v16
 ; GFX6-NEXT:    v_lshrrev_b32_e32 v4, v8, v4
 ; GFX6-NEXT:    v_and_b32_e32 v8, s4, v13
 ; GFX6-NEXT:    v_and_b32_e32 v5, s4, v5
+; GFX6-NEXT:    v_mov_b32_e32 v16, 0xffff
 ; GFX6-NEXT:    v_lshrrev_b32_e32 v5, v8, v5
 ; GFX6-NEXT:    v_and_b32_e32 v8, s4, v14
 ; GFX6-NEXT:    v_and_b32_e32 v6, s4, v6
-; GFX6-NEXT:    v_and_b32_e32 v0, v0, v16
-; GFX6-NEXT:    v_lshlrev_b32_e32 v1, 16, v1
 ; GFX6-NEXT:    v_lshrrev_b32_e32 v6, v8, v6
-; GFX6-NEXT:    v_or_b32_e32 v0, v0, v1
-; GFX6-NEXT:    v_and_b32_e32 v1, v2, v16
-; GFX6-NEXT:    v_and_b32_e32 v2, v3, v16
+; GFX6-NEXT:    v_lshlrev_b32_e32 v1, 16, v1
 ; GFX6-NEXT:    v_and_b32_e32 v8, v15, v16
 ; GFX6-NEXT:    v_and_b32_e32 v7, v7, v16
-; GFX6-NEXT:    v_and_b32_e32 v3, v5, v16
-; GFX6-NEXT:    v_lshlrev_b32_e32 v2, 16, v2
+; GFX6-NEXT:    v_or_b32_e32 v0, v0, v1
+; GFX6-NEXT:    v_lshlrev_b32_e32 v1, 16, v3
 ; GFX6-NEXT:    v_lshrrev_b32_e32 v7, v8, v7
-; GFX6-NEXT:    v_or_b32_e32 v1, v1, v2
-; GFX6-NEXT:    v_and_b32_e32 v2, v4, v16
-; GFX6-NEXT:    v_and_b32_e32 v4, v7, v16
-; GFX6-NEXT:    v_lshlrev_b32_e32 v3, 16, v3
-; GFX6-NEXT:    v_or_b32_e32 v2, v2, v3
-; GFX6-NEXT:    v_and_b32_e32 v3, v6, v16
-; GFX6-NEXT:    v_lshlrev_b32_e32 v4, 16, v4
-; GFX6-NEXT:    v_or_b32_e32 v3, v3, v4
+; GFX6-NEXT:    v_lshlrev_b32_e32 v3, 16, v7
+; GFX6-NEXT:    v_or_b32_e32 v1, v2, v1
+; GFX6-NEXT:    v_lshlrev_b32_e32 v2, 16, v5
+; GFX6-NEXT:    v_or_b32_e32 v2, v4, v2
+; GFX6-NEXT:    v_or_b32_e32 v3, v6, v3
 ; GFX6-NEXT:    s_setpc_b64 s[30:31]
 ;
 ; GFX8-LABEL: v_lshr_v8i16:
 ; GFX8:       ; %bb.0:
 ; GFX8-NEXT:    s_waitcnt vmcnt(0) expcnt(0) lgkmcnt(0)
 ; GFX8-NEXT:    v_lshrrev_b16_e32 v8, v4, v0
 ; GFX8-NEXT:    v_lshrrev_b16_sdwa v0, v4, v0 dst_sel:WORD_1 dst_unused:UNUSED_PAD src0_sel:WORD_1 src1_sel:WORD_1
 ; GFX8-NEXT:    v_lshrrev_b16_e32 v4, v5, v1

 ; GFX6-NEXT:    s_and_b32 s8, s10, s16
 ; GFX6-NEXT:    s_and_b32 s2, s2, s16
 ; GFX6-NEXT:    s_lshr_b32 s2, s2, s8
 ; GFX6-NEXT:    s_and_b32 s8, s11, s16
 ; GFX6-NEXT:    s_and_b32 s3, s3, s16
 ; GFX6-NEXT:    s_lshr_b32 s3, s3, s8
 ; GFX6-NEXT:    s_and_b32 s8, s12, s16
 ; GFX6-NEXT:    s_and_b32 s4, s4, s16
-; GFX6-NEXT:    s_and_b32 s1, s1, s16
 ; GFX6-NEXT:    s_lshr_b32 s4, s4, s8
 ; GFX6-NEXT:    s_and_b32 s8, s13, s16
 ; GFX6-NEXT:    s_and_b32 s5, s5, s16
 ; GFX6-NEXT:    s_lshr_b32 s5, s5, s8
 ; GFX6-NEXT:    s_and_b32 s8, s14, s16
 ; GFX6-NEXT:    s_and_b32 s6, s6, s16
-; GFX6-NEXT:    s_and_b32 s0, s0, s16
-; GFX6-NEXT:    s_lshl_b32 s1, s1, 16
 ; GFX6-NEXT:    s_lshr_b32 s6, s6, s8
-; GFX6-NEXT:    s_or_b32 s0, s0, s1
-; GFX6-NEXT:    s_and_b32 s1, s2, s16
-; GFX6-NEXT:    s_and_b32 s2, s3, s16
+; GFX6-NEXT:    s_lshl_b32 s1, s1, 16
 ; GFX6-NEXT:    s_and_b32 s8, s15, s16
 ; GFX6-NEXT:    s_and_b32 s7, s7, s16
-; GFX6-NEXT:    s_and_b32 s3, s5, s16
-; GFX6-NEXT:    s_lshl_b32 s2, s2, 16
+; GFX6-NEXT:    s_or_b32 s0, s0, s1
+; GFX6-NEXT:    s_lshl_b32 s1, s3, 16
 ; GFX6-NEXT:    s_lshr_b32 s7, s7, s8
-; GFX6-NEXT:    s_or_b32 s1, s1, s2
-; GFX6-NEXT:    s_and_b32 s2, s4, s16
-; GFX6-NEXT:    s_and_b32 s4, s7, s16
-; GFX6-NEXT:    s_lshl_b32 s3, s3, 16
-; GFX6-NEXT:    s_or_b32 s2, s2, s3
-; GFX6-NEXT:    s_and_b32 s3, s6, s16
-; GFX6-NEXT:    s_lshl_b32 s4, s4, 16
-; GFX6-NEXT:    s_or_b32 s3, s3, s4
+; GFX6-NEXT:    s_lshl_b32 s3, s7, 16
+; GFX6-NEXT:    s_or_b32 s1, s2, s1
+; GFX6-NEXT:    s_lshl_b32 s2, s5, 16
+; GFX6-NEXT:    s_or_b32 s2, s4, s2
+; GFX6-NEXT:    s_or_b32 s3, s6, s3
 ; GFX6-NEXT:    ; return to shader part epilog
 ;
 ; GFX8-LABEL: s_lshr_v8i16:
 ; GFX8:       ; %bb.0:
 ; GFX8-NEXT:    s_mov_b32 s12, 0xffff
 ; GFX8-NEXT:    s_lshr_b32 s8, s0, 16
 ; GFX8-NEXT:    s_lshr_b32 s13, s4, 16
 ; GFX8-NEXT:    s_and_b32 s0, s0, s12