Lower X%C into X/C+stuff. This allows the 'division by a constant' logic to

apply to rems as well as divs.  This fixes PR945 and speeds up ReedSolomon
from 14.57s to 10.90s (which is now faster than gcc).

It compiles CodeGen/X86/rem.ll into:

_test1:
        subl $4, %esp
        movl %esi, (%esp)
        movl $2155905153, %ecx
        movl 8(%esp), %esi
        movl %esi, %eax
        imull %ecx
        addl %esi, %edx
        movl %edx, %eax
        shrl $31, %eax
        sarl $7, %edx
        addl %eax, %edx
        imull $255, %edx, %eax
        subl %eax, %esi
        movl %esi, %eax
        movl (%esp), %esi
        addl $4, %esp
        ret
_test2:
        movl 4(%esp), %eax
        movl %eax, %ecx
        sarl $31, %ecx
        shrl $24, %ecx
        addl %eax, %ecx
        andl $4294967040, %ecx
        subl %ecx, %eax
        ret
_test3:
        subl $4, %esp
        movl %esi, (%esp)
        movl $2155905153, %ecx
        movl 8(%esp), %esi
        movl %esi, %eax
        mull %ecx
        shrl $7, %edx
        imull $255, %edx, %eax
        subl %eax, %esi
        movl %esi, %eax
        movl (%esp), %esi
        addl $4, %esp
        ret

instead of div/idiv instructions.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@30920 91177308-0d34-0410-b5e6-96231b3b80d8

lib/CodeGen/SelectionDAG/DAGCombiner.cpp

@@ -885,6 +885,18 @@ SDOperand DAGCombiner::visitSREM(SDNode *N) {
   if (TLI.MaskedValueIsZero(N1, SignBit) &&
       TLI.MaskedValueIsZero(N0, SignBit))
     return DAG.getNode(ISD::UREM, VT, N0, N1);
+  
+  // Unconditionally lower X%C -> X-X/C*C.  This allows the X/C logic to hack on
+  // the remainder operation.
+  if (N1C && !N1C->isNullValue()) {
+    SDOperand Div = DAG.getNode(ISD::SDIV, VT, N0, N1);
+    SDOperand Mul = DAG.getNode(ISD::MUL, VT, Div, N1);
+    SDOperand Sub = DAG.getNode(ISD::SUB, VT, N0, Mul);
+    AddToWorkList(Div.Val);
+    AddToWorkList(Mul.Val);
+    return Sub;
+  }
+  
   return SDOperand();
 }
 
@@ -911,6 +923,18 @@ SDOperand DAGCombiner::visitUREM(SDNode *N) {
       }
     }
   }
+  
+  // Unconditionally lower X%C -> X-X/C*C.  This allows the X/C logic to hack on
+  // the remainder operation.
+  if (N1C && !N1C->isNullValue()) {
+    SDOperand Div = DAG.getNode(ISD::UDIV, VT, N0, N1);
+    SDOperand Mul = DAG.getNode(ISD::MUL, VT, Div, N1);
+    SDOperand Sub = DAG.getNode(ISD::SUB, VT, N0, Mul);
+    AddToWorkList(Div.Val);
+    AddToWorkList(Mul.Val);
+    return Sub;
+  }
+  
   return SDOperand();
 }