[llvm] [GlobalISel] Handle div-by-pow2 (PR #83155)
Shilei Tian via llvm-commits
llvm-commits at lists.llvm.org
Wed Mar 6 16:11:42 PST 2024
================
@@ -5286,6 +5286,156 @@ MachineInstr *CombinerHelper::buildSDivUsingMul(MachineInstr &MI) {
return MIB.buildMul(Ty, Res, Factor);
}
+bool CombinerHelper::matchDivByPow2(MachineInstr &MI, bool IsSigned) {
+ assert((MI.getOpcode() == TargetOpcode::G_SDIV ||
+ MI.getOpcode() == TargetOpcode::G_UDIV) &&
+ "Expected SDIV or UDIV");
+ auto &Div = cast<GenericMachineInstr>(MI);
+ Register RHS = Div.getReg(2);
+ auto MatchPow2 = [&](const Constant *C) {
+ if (auto *CI = dyn_cast<ConstantInt>(C)) {
+ if (CI->getValue().isPowerOf2())
+ return true;
+ if (IsSigned && CI->getValue().isNegatedPowerOf2())
+ return true;
+ }
+ return false;
+ };
+ return matchUnaryPredicate(MRI, RHS, MatchPow2, /*AllowUndefs=*/false);
+}
+
+void CombinerHelper::applySDivByPow2(MachineInstr &MI) {
+ assert(MI.getOpcode() == TargetOpcode::G_SDIV && "Expected SDIV");
+ auto &SDiv = cast<GenericMachineInstr>(MI);
+ Register Dst = SDiv.getReg(0);
+ Register LHS = SDiv.getReg(1);
+ Register RHS = SDiv.getReg(2);
+ LLT Ty = MRI.getType(Dst);
+ LLT ShiftAmtTy = getTargetLowering().getPreferredShiftAmountTy(Ty);
+
+ Builder.setInstrAndDebugLoc(MI);
+
+ // Effectively we want to lower G_SDIV %lhs, %rhs, where %rhs is a power of 2,
+ // to the following version:
+ //
+ // %bits = G_CONSTANT $bitwidth
+ // %c1 = G_CTTZ %rhs
+ // %c1 = G_ZEXT %c1
+ // %inexact = G_SUB $bits, %c1
+ // %tmp = G_CONSTANT ($bitwidth - 1)
+ // %sign = %G_ASHR %lhs, %tmp
+ // %srl = G_SHL %sign, %inexact
+ // %add = G_ADD $lhs, $srl
+ // $sra = G_ASHR %add, %c1
+ // %one = G_CONSTANT $1
+ // %allones = G_CONSTANT %111..1
+ // %isone = G_ICMP EQ %rhs, $one
+ // %isallones = G_ICMP EQ %rhs, $allones
+ // %isoneorallones = G_OR %isone, $isallones
+ // %sra = G_SELECT, %isoneorallones, %lhs, %sra
+ // %zero = G_CONSTANT $0
+ // %sub = G_SUB %zero, %sra
+ // %isneg = G_ICMP SLT $lhs, %zero
+ // %res = G_SELECT %isneg, %sub, %sra
+ //
+ // When $rhs is a constant integer, or a splat vector, we can check its value
+ // at compile time such that the first two G_ICMP conditional statements, as
+ // well as the corresponding non-taken branches, can be eliminated. This can
+ // generate compact code even w/o any constant folding afterwards. When $rhs
+ // is not a splat vector, we have to generate those checks via instructions.
+
+ unsigned Bitwidth = Ty.getScalarSizeInBits();
+ auto Zero = Builder.buildConstant(Ty, 0);
+
+ if (auto RHSC = getConstantOrConstantSplatVector(RHS)) {
----------------
shiltian wrote:
TBH I think this is not necessary if we can improve the constant folding.
https://github.com/llvm/llvm-project/pull/83155
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