[SelectionDAG] Improve knownbits handling of UMIN/UMAX (PR31293)

This patch improves the knownbits logic for unsigned integer min/max opcodes.

For UMIN we know that the result will have the maximum of the inputs' known leading zero bits in the result, similarly for UMAX the maximum of the inputs' leading one bits.

This is particularly useful for simplifying clamping patterns,. e.g. as SSE doesn't have a uitofp instruction we want to use sitofp instead where possible and for that we need to confirm that the top bit is not set.

Differential Revision: https://reviews.llvm.org/D28853

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@292528 91177308-0d34-0410-b5e6-96231b3b80d8

diff --git a/lib/CodeGen/SelectionDAG/SelectionDAG.cpp b/lib/CodeGen/SelectionDAG/SelectionDAG.cpp
index be3eef9234b8df926100e319b9f72d27624fce83..d7b4b4291a1bee9d056400bf70051a84b731cbf8 100644 (file)
--- a/lib/CodeGen/SelectionDAG/SelectionDAG.cpp
+++ b/lib/CodeGen/SelectionDAG/SelectionDAG.cpp
@@ -2687,10 +2687,40 @@ void SelectionDAG::computeKnownBits(SDValue Op, APInt &KnownZero,
     KnownOne = KnownOne2.byteSwap();
     break;
   }
-  case ISD::SMIN:
-  case ISD::SMAX:
-  case ISD::UMIN:
+  case ISD::UMIN: {
+    computeKnownBits(Op.getOperand(0), KnownZero, KnownOne, DemandedElts,
+                     Depth + 1);
+    computeKnownBits(Op.getOperand(1), KnownZero2, KnownOne2, DemandedElts,
+                     Depth + 1);
+
+    // UMIN - we know that the result will have the maximum of the
+    // known zero leading bits of the inputs.
+    unsigned LeadZero = KnownZero.countLeadingOnes();
+    LeadZero = std::max(LeadZero, KnownZero2.countLeadingOnes());
+
+    KnownZero &= KnownZero2;
+    KnownOne &= KnownOne2;
+    KnownZero |= APInt::getHighBitsSet(BitWidth, LeadZero);
+    break;
+  }
   case ISD::UMAX: {
+    computeKnownBits(Op.getOperand(0), KnownZero, KnownOne, DemandedElts,
+                     Depth + 1);
+    computeKnownBits(Op.getOperand(1), KnownZero2, KnownOne2, DemandedElts,
+                     Depth + 1);
+
+    // UMAX - we know that the result will have the maximum of the
+    // known one leading bits of the inputs.
+    unsigned LeadOne = KnownOne.countLeadingOnes();
+    LeadOne = std::max(LeadOne, KnownOne2.countLeadingOnes());
+
+    KnownZero &= KnownZero2;
+    KnownOne &= KnownOne2;
+    KnownOne |= APInt::getHighBitsSet(BitWidth, LeadOne);
+    break;
+  }
+  case ISD::SMIN:
+  case ISD::SMAX: {
     computeKnownBits(Op.getOperand(0), KnownZero, KnownOne, DemandedElts,
                      Depth + 1);
     // If we don't know any bits, early out.

diff --git a/test/CodeGen/X86/known-bits-vector.ll b/test/CodeGen/X86/known-bits-vector.ll
index 19b2811409205b88005cbd0ab92132ce428cb8f8..77e3c537dfe70206c395061986da192cd62ad225 100644 (file)
--- a/test/CodeGen/X86/known-bits-vector.ll
+++ b/test/CodeGen/X86/known-bits-vector.ll
@@ -481,22 +481,14 @@ define <4 x float> @knownbits_umin_shuffle_uitofp(<4 x i32> %a0) {
 ; X32:       # BB#0:
 ; X32-NEXT:    vpminud {{\.LCPI.*}}, %xmm0, %xmm0
 ; X32-NEXT:    vpshufd {{.*#+}} xmm0 = xmm0[0,0,3,3]
-; X32-NEXT:    vpblendw {{.*#+}} xmm1 = xmm0[0],mem[1],xmm0[2],mem[3],xmm0[4],mem[5],xmm0[6],mem[7]
-; X32-NEXT:    vpsrld $16, %xmm0, %xmm0
-; X32-NEXT:    vpblendw {{.*#+}} xmm0 = xmm0[0],mem[1],xmm0[2],mem[3],xmm0[4],mem[5],xmm0[6],mem[7]
-; X32-NEXT:    vaddps {{\.LCPI.*}}, %xmm0, %xmm0
-; X32-NEXT:    vaddps %xmm0, %xmm1, %xmm0
+; X32-NEXT:    vcvtdq2ps %xmm0, %xmm0
 ; X32-NEXT:    retl
 ;
 ; X64-LABEL: knownbits_umin_shuffle_uitofp:
 ; X64:       # BB#0:
 ; X64-NEXT:    vpminud {{.*}}(%rip), %xmm0, %xmm0
 ; X64-NEXT:    vpshufd {{.*#+}} xmm0 = xmm0[0,0,3,3]
-; X64-NEXT:    vpblendw {{.*#+}} xmm1 = xmm0[0],mem[1],xmm0[2],mem[3],xmm0[4],mem[5],xmm0[6],mem[7]
-; X64-NEXT:    vpsrld $16, %xmm0, %xmm0
-; X64-NEXT:    vpblendw {{.*#+}} xmm0 = xmm0[0],mem[1],xmm0[2],mem[3],xmm0[4],mem[5],xmm0[6],mem[7]
-; X64-NEXT:    vaddps {{.*}}(%rip), %xmm0, %xmm0
-; X64-NEXT:    vaddps %xmm0, %xmm1, %xmm0
+; X64-NEXT:    vcvtdq2ps %xmm0, %xmm0
 ; X64-NEXT:    retq
   %1 = call <4 x i32> @llvm.x86.sse41.pminud(<4 x i32> %a0, <4 x i32> <i32 65535, i32 -1, i32 -1, i32 262143>)
   %2 = shufflevector <4 x i32> %1, <4 x i32> undef, <4 x i32> <i32 0, i32 0, i32 3, i32 3>
@@ -511,14 +503,12 @@ define <4 x i32> @knownbits_umax_shuffle_ashr(<4 x i32> %a0) {
 ; X32:       # BB#0:
 ; X32-NEXT:    vpmaxud {{\.LCPI.*}}, %xmm0, %xmm0
 ; X32-NEXT:    vpshufd {{.*#+}} xmm0 = xmm0[1,1,2,2]
-; X32-NEXT:    vpsrad $31, %xmm0, %xmm0
 ; X32-NEXT:    retl
 ;
 ; X64-LABEL: knownbits_umax_shuffle_ashr:
 ; X64:       # BB#0:
 ; X64-NEXT:    vpmaxud {{.*}}(%rip), %xmm0, %xmm0
 ; X64-NEXT:    vpshufd {{.*#+}} xmm0 = xmm0[1,1,2,2]
-; X64-NEXT:    vpsrad $31, %xmm0, %xmm0
 ; X64-NEXT:    retq
   %1 = call <4 x i32> @llvm.x86.sse41.pmaxud(<4 x i32> %a0, <4 x i32> <i32 65535, i32 -1, i32 -1, i32 262143>)
   %2 = shufflevector <4 x i32> %1, <4 x i32> undef, <4 x i32> <i32 1, i32 1, i32 2, i32 2>