[PATCH] D88429: [InstCombine] visitTrunc - trunc (*shr (trunc A), C) --> trunc(*shr A, C)

[Simon Pilgrim via Phabricator via llvm-commits]

RKSimon created this revision.
RKSimon added reviewers: spatel, nikic, lebedev.ri, reames.
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Attempt to fold trunc (*shr (trunc A), C) --> trunc(*shr A, C) iff the shift amount if small enough that all zero/sign bits created by the shift are removed by the last trunc.

Helps fix the regressions encountered in D88316 <https://reviews.llvm.org/D88316>.


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D88429

Files:
  llvm/lib/Transforms/InstCombine/InstCombineCasts.cpp
  llvm/test/Transforms/InstCombine/trunc-shift-trunc.ll


Index: llvm/test/Transforms/InstCombine/trunc-shift-trunc.ll
===================================================================
--- llvm/test/Transforms/InstCombine/trunc-shift-trunc.ll
+++ llvm/test/Transforms/InstCombine/trunc-shift-trunc.ll
@@ -9,9 +9,8 @@
 
 define i8 @trunc_lshr_trunc(i64 %a) {
 ; CHECK-LABEL: @trunc_lshr_trunc(
-; CHECK-NEXT:    [[B:%.*]] = trunc i64 [[A:%.*]] to i32
-; CHECK-NEXT:    [[C:%.*]] = lshr i32 [[B]], 8
-; CHECK-NEXT:    [[D:%.*]] = trunc i32 [[C]] to i8
+; CHECK-NEXT:    [[TMP1:%.*]] = lshr i64 [[A:%.*]], 8
+; CHECK-NEXT:    [[D:%.*]] = trunc i64 [[TMP1]] to i8
 ; CHECK-NEXT:    ret i8 [[D]]
 ;
   %b = trunc i64 %a to i32
@@ -22,9 +21,8 @@
 
 define <2 x i8> @trunc_lshr_trunc_uniform(<2 x i64> %a) {
 ; CHECK-LABEL: @trunc_lshr_trunc_uniform(
-; CHECK-NEXT:    [[B:%.*]] = trunc <2 x i64> [[A:%.*]] to <2 x i32>
-; CHECK-NEXT:    [[C:%.*]] = lshr <2 x i32> [[B]], <i32 8, i32 8>
-; CHECK-NEXT:    [[D:%.*]] = trunc <2 x i32> [[C]] to <2 x i8>
+; CHECK-NEXT:    [[TMP1:%.*]] = lshr <2 x i64> [[A:%.*]], <i64 8, i64 8>
+; CHECK-NEXT:    [[D:%.*]] = trunc <2 x i64> [[TMP1]] to <2 x i8>
 ; CHECK-NEXT:    ret <2 x i8> [[D]]
 ;
   %b = trunc <2 x i64> %a to <2 x i32>
@@ -61,9 +59,8 @@
 
 define i8 @trunc_ashr_trunc(i64 %a) {
 ; CHECK-LABEL: @trunc_ashr_trunc(
-; CHECK-NEXT:    [[B:%.*]] = trunc i64 [[A:%.*]] to i32
-; CHECK-NEXT:    [[TMP1:%.*]] = lshr i32 [[B]], 8
-; CHECK-NEXT:    [[D:%.*]] = trunc i32 [[TMP1]] to i8
+; CHECK-NEXT:    [[TMP1:%.*]] = lshr i64 [[A:%.*]], 8
+; CHECK-NEXT:    [[D:%.*]] = trunc i64 [[TMP1]] to i8
 ; CHECK-NEXT:    ret i8 [[D]]
 ;
   %b = trunc i64 %a to i32
@@ -74,9 +71,8 @@
 
 define <2 x i8> @trunc_ashr_trunc_uniform(<2 x i64> %a) {
 ; CHECK-LABEL: @trunc_ashr_trunc_uniform(
-; CHECK-NEXT:    [[B:%.*]] = trunc <2 x i64> [[A:%.*]] to <2 x i32>
-; CHECK-NEXT:    [[TMP1:%.*]] = lshr <2 x i32> [[B]], <i32 8, i32 8>
-; CHECK-NEXT:    [[D:%.*]] = trunc <2 x i32> [[TMP1]] to <2 x i8>
+; CHECK-NEXT:    [[TMP1:%.*]] = lshr <2 x i64> [[A:%.*]], <i64 8, i64 8>
+; CHECK-NEXT:    [[D:%.*]] = trunc <2 x i64> [[TMP1]] to <2 x i8>
 ; CHECK-NEXT:    ret <2 x i8> [[D]]
 ;
   %b = trunc <2 x i64> %a to <2 x i32>
Index: llvm/lib/Transforms/InstCombine/InstCombineCasts.cpp
===================================================================
--- llvm/lib/Transforms/InstCombine/InstCombineCasts.cpp
+++ llvm/lib/Transforms/InstCombine/InstCombineCasts.cpp
@@ -851,6 +851,23 @@
     // TODO: Mask high bits with 'and'.
   }
 
+  // trunc (*shr (trunc A), C) --> trunc(*shr A, C)
+  if (match(Src, m_OneUse(m_Shr(m_Trunc(m_Value(A)), m_APInt(C))))) {
+    Type *AType = A->getType();
+    unsigned AWidth = AType->getScalarSizeInBits();
+    unsigned MaxShiftAmt = std::min<unsigned>(DestWidth, AWidth - DestWidth);
+
+    // If the shift is small enough, all zero/sign bits created by the shift are
+    // removed by the trunc.
+    if (C->ule(MaxShiftAmt)) {
+      auto *ShAmt = ConstantInt::get(AType, C->getZExtValue());
+      Value *Shift = cast<Instruction>(Src)->getOpcode() == Instruction::AShr
+                         ? Builder.CreateAShr(A, ShAmt)
+                         : Builder.CreateLShr(A, ShAmt);
+      return CastInst::CreateTruncOrBitCast(Shift, DestTy);
+    }
+  }
+
   if (Instruction *I = narrowBinOp(Trunc))
     return I;
 


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