[GlobalISel]: Simplify udiv lowering by determining known zeros

Author: AZero13

llvm/lib/CodeGen/GlobalISel/CombinerHelper.cpp

@@ -5069,6 +5069,9 @@ MachineInstr *CombinerHelper::buildUDivUsingMul(MachineInstr &MI) {
   const unsigned EltBits = ScalarTy.getScalarSizeInBits();
   LLT ShiftAmtTy = getTargetLowering().getPreferredShiftAmountTy(Ty);
   LLT ScalarShiftAmtTy = ShiftAmtTy.getScalarType();
+
+  unsigned KnownLeadingZeros =
+      KB ? KB->getKnownBits(LHS).countMinLeadingZeros() : 0;
   auto &MIB = Builder;
 
   bool UseNPQ = false;
@@ -5086,8 +5089,12 @@ MachineInstr *CombinerHelper::buildUDivUsingMul(MachineInstr &MI) {
     // at the end.
     // TODO: Use undef values for divisor of 1.
     if (!Divisor.isOne()) {
+
+      // UnsignedDivisionByConstantInfo doesn't work correctly if leading zeros
+      // in the dividend exceeds the leading zeros for the divisor.
       UnsignedDivisionByConstantInfo magics =
-          UnsignedDivisionByConstantInfo::get(Divisor);
+          UnsignedDivisionByConstantInfo::get(
+              Divisor, std::min(KnownLeadingZeros, Divisor.countl_zero()));
 
       Magic = std::move(magics.Magic);
 

llvm/test/CodeGen/AArch64/GlobalISel/combine-udiv.ll

@@ -257,12 +257,12 @@ define i32 @udiv_div_by_180(i32 %x)
 ;
 ; GISEL-LABEL: udiv_div_by_180:
 ; GISEL:       // %bb.0:
-; GISEL-NEXT:    ubfx w8, w0, #2, #6
-; GISEL-NEXT:    mov w9, #27671 // =0x6c17
-; GISEL-NEXT:    movk w9, #5825, lsl #16
+; GISEL-NEXT:    uxtb w8, w0
+; GISEL-NEXT:    mov w9, #5826 // =0x16c2
+; GISEL-NEXT:    movk w9, #364, lsl #16
 ; GISEL-NEXT:    umull x8, w8, w9
-; GISEL-NEXT:    lsr x8, x8, #32
-; GISEL-NEXT:    lsr w0, w8, #2
+; GISEL-NEXT:    lsr x0, x8, #32
+; GISEL-NEXT:    // kill: def $w0 killed $w0 killed $x0
 ; GISEL-NEXT:    ret
 {
   %truncate = and i32 %x, 255