[llvm] e2a0f51 - [InstSimplify] fix potential miscompile in select value equivalence

[Sanjay Patel via llvm-commits]

Author: Sanjay Patel
Date: 2021-04-05T16:52:34-04:00
New Revision: e2a0f512eacad0699be9660f668726d7deb2cd75

URL: https://github.com/llvm/llvm-project/commit/e2a0f512eacad0699be9660f668726d7deb2cd75
DIFF: https://github.com/llvm/llvm-project/commit/e2a0f512eacad0699be9660f668726d7deb2cd75.diff

LOG: [InstSimplify] fix potential miscompile in select value equivalence

This is the sibling fix to c590a9880d7a -
as there, we can't subsitute a vector value the equality
compare replacement that we are trying requires that the
comparison is true for the entire value. Vector select
can be partly true/false.

Added: 
    

Modified: 
    llvm/lib/Analysis/InstructionSimplify.cpp
    llvm/test/Transforms/InstSimplify/select.ll

Removed: 
    


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diff  --git a/llvm/lib/Analysis/InstructionSimplify.cpp b/llvm/lib/Analysis/InstructionSimplify.cpp
index 23bb5e654592..5804f6868052 100644
--- a/llvm/lib/Analysis/InstructionSimplify.cpp
+++ b/llvm/lib/Analysis/InstructionSimplify.cpp
@@ -4149,10 +4149,12 @@ static Value *simplifySelectWithICmpCond(Value *CondVal, Value *TrueVal,
                                               TrueVal, FalseVal))
     return V;
 
-  // If we have an equality comparison, then we know the value in one of the
-  // arms of the select. See if substituting this value into the arm and
+  // If we have a scalar equality comparison, then we know the value in one of
+  // the arms of the select. See if substituting this value into the arm and
   // simplifying the result yields the same value as the other arm.
-  if (Pred == ICmpInst::ICMP_EQ) {
+  // Note that the equivalence/replacement opportunity does not hold for vectors
+  // because each element of a vector select is chosen independently.
+  if (Pred == ICmpInst::ICMP_EQ && !CondVal->getType()->isVectorTy()) {
     if (SimplifyWithOpReplaced(FalseVal, CmpLHS, CmpRHS, Q,
                                /* AllowRefinement */ false, MaxRecurse) ==
             TrueVal ||

diff  --git a/llvm/test/Transforms/InstSimplify/select.ll b/llvm/test/Transforms/InstSimplify/select.ll
index 00cb5c1d5ef3..acb998b8518a 100644
--- a/llvm/test/Transforms/InstSimplify/select.ll
+++ b/llvm/test/Transforms/InstSimplify/select.ll
@@ -1029,7 +1029,10 @@ declare i32 @llvm.ctpop.i32(i32)
 
 define <2 x i32> @vec_select_no_equivalence(<2 x i32> %x, <2 x i32> %y) {
 ; CHECK-LABEL: @vec_select_no_equivalence(
-; CHECK-NEXT:    ret <2 x i32> zeroinitializer
+; CHECK-NEXT:    [[X10:%.*]] = shufflevector <2 x i32> [[X:%.*]], <2 x i32> undef, <2 x i32> <i32 1, i32 0>
+; CHECK-NEXT:    [[COND:%.*]] = icmp eq <2 x i32> [[X]], zeroinitializer
+; CHECK-NEXT:    [[S:%.*]] = select <2 x i1> [[COND]], <2 x i32> [[X10]], <2 x i32> zeroinitializer
+; CHECK-NEXT:    ret <2 x i32> [[S]]
 ;
   %x10 = shufflevector <2 x i32> %x, <2 x i32> undef, <2 x i32> <i32 1, i32 0>
   %cond = icmp eq <2 x i32> %x, zeroinitializer