[InstCombine] Fix PR24605.

PR24605 is caused due to an incorrect insert point in instcombine's IR
builder.  When simplifying

  %t = add X Y
  ...
  %m = icmp ... %t

the replacement for %t should be placed before %t, not before %m, as
there could be a use of %t between %t and %m.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@246315 91177308-0d34-0410-b5e6-96231b3b80d8

lib/Transforms/InstCombine/InstCombineCompares.cpp

@@ -2140,6 +2140,12 @@ bool InstCombiner::OptimizeOverflowCheck(OverflowCheckFlavor OCF, Value *LHS,
     return true;
   };
 
+  // If the overflow check was an add followed by a compare, the insertion point
+  // may be pointing to the compare.  We want to insert the new instructions
+  // before the add in case there are uses of the add between the add and the
+  // compare.
+  Builder->SetInsertPoint(&OrigI);
+
   switch (OCF) {
   case OCF_INVALID:
     llvm_unreachable("bad overflow check kind!");

lib/Transforms/InstCombine/InstCombineInternal.h

@@ -360,6 +360,11 @@ private:
   /// \brief Try to optimize a sequence of instructions checking if an operation
   /// on LHS and RHS overflows.
   ///
+  /// If this overflow check is done via one of the overflow check intrinsics,
+  /// then CtxI has to be the call instruction calling that intrinsic.  If this
+  /// overflow check is done by arithmetic followed by a compare, then CtxI has
+  /// to be the arithmetic instruction.
+  ///
   /// If a simplification is possible, stores the simplified result of the
   /// operation in OperationResult and result of the overflow check in
   /// OverflowResult, and return true.  If no simplification is possible,

test/Transforms/InstCombine/pr24605.ll

@@ -0,0 +1,15 @@
+; RUN: opt -S -instcombine < %s | FileCheck %s
+
+target datalayout = "e-m:e-i64:64-f80:128-n8:16:32:64-S128"
+target triple = "x86_64-unknown-linux-gnu"
+
+define i1 @f(i8* %a, i8 %b) {
+; CHECK-LABEL: @f(
+entry:
+  %or = or i8 %b, -117
+  %sub = add i8 %or, -1
+  store i8 %sub, i8* %a, align 1
+  %cmp = icmp ugt i8 %or, %sub
+  ret i1 %cmp
+; CHECK: ret i1 true
+}