[PATCH] D122340: [InstCombine] SimplifyDemandedUseBits - remove ashr node if we only demand known sign bits

[Simon Pilgrim via Phabricator via llvm-commits]

RKSimon created this revision.
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We already do this for SelectionDAG, but we're missing it here.

Noticed while re-triaging PR21929


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D122340

Files:
  llvm/lib/Transforms/InstCombine/InstCombineSimplifyDemanded.cpp
  llvm/test/Transforms/InstCombine/ashr-demand.ll


Index: llvm/test/Transforms/InstCombine/ashr-demand.ll
===================================================================
--- llvm/test/Transforms/InstCombine/ashr-demand.ll
+++ llvm/test/Transforms/InstCombine/ashr-demand.ll
@@ -5,9 +5,8 @@
 
 define i32 @srem2_ashr_mask(i32 %a0) {
 ; CHECK-LABEL: @srem2_ashr_mask(
-; CHECK-NEXT:    [[TMP1:%.*]] = and i32 [[A0:%.*]], -2147483647
-; CHECK-NEXT:    [[ISNEG:%.*]] = icmp eq i32 [[TMP1]], -2147483647
-; CHECK-NEXT:    [[MASK:%.*]] = select i1 [[ISNEG]], i32 2, i32 0
+; CHECK-NEXT:    [[SREM:%.*]] = srem i32 [[A0:%.*]], 2
+; CHECK-NEXT:    [[MASK:%.*]] = and i32 [[SREM]], 2
 ; CHECK-NEXT:    ret i32 [[MASK]]
 ;
   %srem = srem i32 %a0, 2 ; result = (1,0,-1) num signbits = 31
@@ -31,9 +30,8 @@
 
 define <2 x i32> @srem2_ashr_mask_vector(<2 x i32> %a0) {
 ; CHECK-LABEL: @srem2_ashr_mask_vector(
-; CHECK-NEXT:    [[TMP1:%.*]] = and <2 x i32> [[A0:%.*]], <i32 -2147483647, i32 -2147483647>
-; CHECK-NEXT:    [[ISNEG:%.*]] = icmp eq <2 x i32> [[TMP1]], <i32 -2147483647, i32 -2147483647>
-; CHECK-NEXT:    [[MASK:%.*]] = select <2 x i1> [[ISNEG]], <2 x i32> <i32 2, i32 2>, <2 x i32> zeroinitializer
+; CHECK-NEXT:    [[SREM:%.*]] = srem <2 x i32> [[A0:%.*]], <i32 2, i32 2>
+; CHECK-NEXT:    [[MASK:%.*]] = and <2 x i32> [[SREM]], <i32 2, i32 2>
 ; CHECK-NEXT:    ret <2 x i32> [[MASK]]
 ;
   %srem = srem <2 x i32> %a0, <i32 2, i32 2>
@@ -45,8 +43,7 @@
 define <2 x i32> @srem2_ashr_mask_vector_nonconstant(<2 x i32> %a0, <2 x i32> %a1) {
 ; CHECK-LABEL: @srem2_ashr_mask_vector_nonconstant(
 ; CHECK-NEXT:    [[SREM:%.*]] = srem <2 x i32> [[A0:%.*]], <i32 2, i32 2>
-; CHECK-NEXT:    [[ASHR:%.*]] = ashr <2 x i32> [[SREM]], [[A1:%.*]]
-; CHECK-NEXT:    [[MASK:%.*]] = and <2 x i32> [[ASHR]], <i32 2, i32 2>
+; CHECK-NEXT:    [[MASK:%.*]] = and <2 x i32> [[SREM]], <i32 2, i32 2>
 ; CHECK-NEXT:    ret <2 x i32> [[MASK]]
 ;
   %srem = srem <2 x i32> %a0, <i32 2, i32 2>
Index: llvm/lib/Transforms/InstCombine/InstCombineSimplifyDemanded.cpp
===================================================================
--- llvm/lib/Transforms/InstCombine/InstCombineSimplifyDemanded.cpp
+++ llvm/lib/Transforms/InstCombine/InstCombineSimplifyDemanded.cpp
@@ -637,6 +637,14 @@
     break;
   }
   case Instruction::AShr: {
+    unsigned SignBits = ComputeNumSignBits(I->getOperand(0), Depth + 1, CxtI);
+
+    // If we only want bits that already match the signbit then we don't need
+    // to shift.
+    unsigned NumHiDemandedBits = BitWidth - DemandedMask.countTrailingZeros();
+    if (SignBits >= NumHiDemandedBits)
+      return I->getOperand(0);
+
     // If this is an arithmetic shift right and only the low-bit is set, we can
     // always convert this into a logical shr, even if the shift amount is
     // variable.  The low bit of the shift cannot be an input sign bit unless
@@ -648,11 +656,6 @@
       return InsertNewInstWith(NewVal, *I);
     }
 
-    // If the sign bit is the only bit demanded by this ashr, then there is no
-    // need to do it, the shift doesn't change the high bit.
-    if (DemandedMask.isSignMask())
-      return I->getOperand(0);
-
     const APInt *SA;
     if (match(I->getOperand(1), m_APInt(SA))) {
       uint32_t ShiftAmt = SA->getLimitedValue(BitWidth-1);
@@ -672,8 +675,6 @@
       if (SimplifyDemandedBits(I, 0, DemandedMaskIn, Known, Depth + 1))
         return I;
 
-      unsigned SignBits = ComputeNumSignBits(I->getOperand(0), Depth + 1, CxtI);
-
       assert(!Known.hasConflict() && "Bits known to be one AND zero?");
       // Compute the new bits that are at the top now plus sign bits.
       APInt HighBits(APInt::getHighBitsSet(


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