[llvm] r336562 - [InstCombine] avoid extra poison when moving shift above shuffle

[Sanjay Patel via llvm-commits]

Author: spatel
Date: Mon Jul  9 10:20:20 2018
New Revision: 336562

URL: http://llvm.org/viewvc/llvm-project?rev=336562&view=rev
Log:
[InstCombine] avoid extra poison when moving shift above shuffle

As discussed in D49047 / D48987, shift-by-undef produces poison,
so we can't use undef vector elements in that case..

Note that we need to extend this for poison-generating flags,
and there's a proposal to create poison from FMF in D47963,


Modified:
    llvm/trunk/lib/Transforms/InstCombine/InstructionCombining.cpp
    llvm/trunk/test/Transforms/InstCombine/vec_shuffle.ll

Modified: llvm/trunk/lib/Transforms/InstCombine/InstructionCombining.cpp
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Transforms/InstCombine/InstructionCombining.cpp?rev=336562&r1=336561&r2=336562&view=diff
==============================================================================
--- llvm/trunk/lib/Transforms/InstCombine/InstructionCombining.cpp (original)
+++ llvm/trunk/lib/Transforms/InstCombine/InstructionCombining.cpp Mon Jul  9 10:20:20 2018
@@ -1418,14 +1418,11 @@ Instruction *InstCombiner::foldShuffledB
     }
     if (MayChange) {
       Constant *NewC = ConstantVector::get(NewVecC);
-      // With integer div/rem instructions, it is not safe to use a vector with
-      // undef elements because the entire instruction can be folded to undef.
-      // All other binop opcodes are always safe to speculate, and therefore, it
-      // is fine to include undef elements for unused lanes (and using undefs
-      // may help optimization).
-      // FIXME: This transform is also not poison-safe. Eg, shift-by-undef would
-      // create poison that may not exist in the original code.
-      if (Inst.isIntDivRem())
+      // It may not be safe to execute a binop on a vector with undef elements
+      // because the entire instruction can be folded to undef or create poison
+      // that did not exist in the original code.
+      if (Inst.isIntDivRem() ||
+          (Inst.isShift() && isa<Constant>(Inst.getOperand(1))))
         NewC = getSafeVectorConstantForBinop(Inst.getOpcode(), NewC);
       
       // Op(shuffle(V1, Mask), C) -> shuffle(Op(V1, NewC), Mask)

Modified: llvm/trunk/test/Transforms/InstCombine/vec_shuffle.ll
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/test/Transforms/InstCombine/vec_shuffle.ll?rev=336562&r1=336561&r2=336562&view=diff
==============================================================================
--- llvm/trunk/test/Transforms/InstCombine/vec_shuffle.ll (original)
+++ llvm/trunk/test/Transforms/InstCombine/vec_shuffle.ll Mon Jul  9 10:20:20 2018
@@ -654,7 +654,7 @@ define <2 x i32> @shl_splat_constant0(<2
 
 define <2 x i32> @shl_splat_constant1(<2 x i32> %x) {
 ; CHECK-LABEL: @shl_splat_constant1(
-; CHECK-NEXT:    [[TMP1:%.*]] = shl <2 x i32> [[X:%.*]], <i32 5, i32 undef>
+; CHECK-NEXT:    [[TMP1:%.*]] = shl <2 x i32> [[X:%.*]], <i32 5, i32 0>
 ; CHECK-NEXT:    [[R:%.*]] = shufflevector <2 x i32> [[TMP1]], <2 x i32> undef, <2 x i32> zeroinitializer
 ; CHECK-NEXT:    ret <2 x i32> [[R]]
 ;
@@ -676,7 +676,7 @@ define <2 x i32> @ashr_splat_constant0(<
 
 define <2 x i32> @ashr_splat_constant1(<2 x i32> %x) {
 ; CHECK-LABEL: @ashr_splat_constant1(
-; CHECK-NEXT:    [[TMP1:%.*]] = ashr <2 x i32> [[X:%.*]], <i32 5, i32 undef>
+; CHECK-NEXT:    [[TMP1:%.*]] = ashr <2 x i32> [[X:%.*]], <i32 5, i32 0>
 ; CHECK-NEXT:    [[R:%.*]] = shufflevector <2 x i32> [[TMP1]], <2 x i32> undef, <2 x i32> zeroinitializer
 ; CHECK-NEXT:    ret <2 x i32> [[R]]
 ;
@@ -698,7 +698,7 @@ define <2 x i32> @lshr_splat_constant0(<
 
 define <2 x i32> @lshr_splat_constant1(<2 x i32> %x) {
 ; CHECK-LABEL: @lshr_splat_constant1(
-; CHECK-NEXT:    [[TMP1:%.*]] = lshr <2 x i32> [[X:%.*]], <i32 5, i32 undef>
+; CHECK-NEXT:    [[TMP1:%.*]] = lshr <2 x i32> [[X:%.*]], <i32 5, i32 0>
 ; CHECK-NEXT:    [[R:%.*]] = shufflevector <2 x i32> [[TMP1]], <2 x i32> undef, <2 x i32> zeroinitializer
 ; CHECK-NEXT:    ret <2 x i32> [[R]]
 ;