Fix generic shift expansion when shift amount is 0

Summary:
This fixes http://llvm.org/bugs/show_bug.cgi?id=16439. 

This is one possible way to approach this. The other would be to split InL>>(nbits-Amt) into (InL>>(nbits-1-Amt))>>1, which is also valid since since we only need to care about Amt up nbits-1. It's hard to tell which one is better since the shift might be expensive if this stage of expansion is not yet a legal machine integer, whereas comparisons with zero are relatively cheap at all sizes, but more expensive than a shift if the shift is on a legal machine type. 

Patch by Keno Fischer!

Test Plan: regression test from http://reviews.llvm.org/D7752

Reviewers: chfast, resistor

Reviewed By: chfast, resistor

Subscribers: sanjoy, resistor, chfast, llvm-commits

Differential Revision: http://reviews.llvm.org/D4978

llvm-svn: 235370

llvm/lib/CodeGen/SelectionDAG/LegalizeIntegerTypes.cpp

@@ -1547,6 +1547,9 @@ ExpandShiftWithUnknownAmountBit(SDNode *N, SDValue &Lo, SDValue &Hi) {
   SDValue AmtLack = DAG.getNode(ISD::SUB, dl, ShTy, NVBitsNode, Amt);
   SDValue isShort = DAG.getSetCC(dl, getSetCCResultType(ShTy),
                                  Amt, NVBitsNode, ISD::SETULT);
+  SDValue isZero = DAG.getSetCC(dl, getSetCCResultType(ShTy),
+                                Amt, DAG.getConstant(0, ShTy),
+                                ISD::SETEQ);
 
   SDValue LoS, HiS, LoL, HiL;
   switch (N->getOpcode()) {
@@ -1556,16 +1559,15 @@ ExpandShiftWithUnknownAmountBit(SDNode *N, SDValue &Lo, SDValue &Hi) {
     LoS = DAG.getNode(ISD::SHL, dl, NVT, InL, Amt);
     HiS = DAG.getNode(ISD::OR, dl, NVT,
                       DAG.getNode(ISD::SHL, dl, NVT, InH, Amt),
-    // FIXME: If Amt is zero, the following shift generates an undefined result
-    // on some architectures.
                       DAG.getNode(ISD::SRL, dl, NVT, InL, AmtLack));
 
     // Long: ShAmt >= NVTBits
     LoL = DAG.getConstant(0, NVT);                        // Lo part is zero.
     HiL = DAG.getNode(ISD::SHL, dl, NVT, InL, AmtExcess); // Hi from Lo part.
 
     Lo = DAG.getSelect(dl, NVT, isShort, LoS, LoL);
-    Hi = DAG.getSelect(dl, NVT, isShort, HiS, HiL);
+    Hi = DAG.getSelect(dl, NVT, isZero, InH,
+                       DAG.getSelect(dl, NVT, isShort, HiS, HiL));
     return true;
   case ISD::SRL:
     // Short: ShAmt < NVTBits
@@ -1580,24 +1582,24 @@ ExpandShiftWithUnknownAmountBit(SDNode *N, SDValue &Lo, SDValue &Hi) {
     HiL = DAG.getConstant(0, NVT);                        // Hi part is zero.
     LoL = DAG.getNode(ISD::SRL, dl, NVT, InH, AmtExcess); // Lo from Hi part.
 
-    Lo = DAG.getSelect(dl, NVT, isShort, LoS, LoL);
+    Lo = DAG.getSelect(dl, NVT, isZero, InL,
+                       DAG.getSelect(dl, NVT, isShort, LoS, LoL));
     Hi = DAG.getSelect(dl, NVT, isShort, HiS, HiL);
     return true;
   case ISD::SRA:
     // Short: ShAmt < NVTBits
     HiS = DAG.getNode(ISD::SRA, dl, NVT, InH, Amt);
     LoS = DAG.getNode(ISD::OR, dl, NVT,
                       DAG.getNode(ISD::SRL, dl, NVT, InL, Amt),
-    // FIXME: If Amt is zero, the following shift generates an undefined result
-    // on some architectures.
                       DAG.getNode(ISD::SHL, dl, NVT, InH, AmtLack));
 
     // Long: ShAmt >= NVTBits
     HiL = DAG.getNode(ISD::SRA, dl, NVT, InH,             // Sign of Hi part.
                       DAG.getConstant(NVTBits-1, ShTy));
     LoL = DAG.getNode(ISD::SRA, dl, NVT, InH, AmtExcess); // Lo from Hi part.
 
-    Lo = DAG.getSelect(dl, NVT, isShort, LoS, LoL);
+    Lo = DAG.getSelect(dl, NVT, isZero, InL,
+                       DAG.getSelect(dl, NVT, isShort, LoS, LoL));
     Hi = DAG.getSelect(dl, NVT, isShort, HiS, HiL);
     return true;
   }

llvm/test/CodeGen/X86/shift-i256.ll

@@ -1,9 +1,21 @@
-; RUN: llc < %s -march=x86
-; RUN: llc < %s -march=x86-64
+; RUN: llc < %s -march=x86        | FileCheck %s
+; RUN: llc < %s -march=x86-64 -O0 | FileCheck %s -check-prefix=CHECK-X64
+; RUN: llc < %s -march=x86-64 -O2 | FileCheck %s -check-prefix=CHECK-X64
 
-define void @t(i256 %x, i256 %a, i256* nocapture %r) nounwind readnone {
+; CHECK-LABEL: shift1
+define void @shift1(i256 %x, i256 %a, i256* nocapture %r) nounwind readnone {
 entry:
 	%0 = ashr i256 %x, %a
 	store i256 %0, i256* %r
         ret void
 }
+
+; CHECK-LABEL: shift2
+define i256 @shift2(i256 %c) nounwind
+{
+  %b = shl i256 1, %c  ; %c must not be a constant
+  ; Special case when %c is 0:
+  ; CHECK-X64: testb [[REG:%r[0-9]+b]], [[REG]]
+  ; CHECK-X64: cmoveq
+  ret i256 %b
+}