Teach InstCombine to eliminate malloc-realloc-free triplets.

[resistor]

• Teach InstCombine to remove the realloc from free(realloc(...)).

[majnemer]

LGTM

[resistor]

Squash

llvm/lib/Transforms/InstCombine/InstructionCombining.cpp

for (User *U : PI->users()) {
             continue;
           }
         }
 
         if (isFreeCall(I, TLI)) {
           Users.emplace_back(I);
           continue;
         }
+
+        if (isReallocLikeFn(I, TLI, true)) {
+          Users.emplace_back(I);
+          Worklist.push_back(I);
+          continue;
+        }
+
         return false;
 
       case Instruction::Store: {
         StoreInst *SI = cast<StoreInst>(I);
         if (SI->isVolatile() || SI->getPointerOperand() != PI)
           return false;
         Users.emplace_back(I);
         continue;

llvm/test/Transforms/InstCombine/alloc-realloc-free.ll

+; NOTE: Assertions have been autogenerated by utils/update_test_checks.py UTC_ARGS: --scrub-attributes
+; RUN: opt -S -instcombine < %s | FileCheck %s
+
+define dso_local void @test() local_unnamed_addr #0 {
+; CHECK-LABEL: @test(
+; CHECK-NEXT:    ret void
+;
+  %1 = tail call noalias align 16 dereferenceable_or_null(4) i8* @malloc(i64 4) #4
+  %2 = tail call align 16 dereferenceable_or_null(6) i8* @realloc(i8* %1, i64 6) #4
+  tail call void @free(i8* %2) #4
+  ret void
+}
+
+declare dso_local noalias noundef i8* @malloc(i64 noundef) local_unnamed_addr #1
+declare dso_local noalias noundef i8* @realloc(i8* nocapture, i64 noundef) local_unnamed_addr #2
+declare dso_local void @free(i8* nocapture noundef) local_unnamed_addr #2
+declare void @llvm.dbg.value(metadata, metadata, metadata) #3
+
+attributes #0 = { mustprogress nounwind uwtable willreturn }
+attributes #1 = { inaccessiblememonly mustprogress nofree nounwind willreturn }
+attributes #2 = { inaccessiblemem_or_argmemonly mustprogress nounwind willreturn }
+attributes #3 = { nofree nosync nounwind readnone speculatable willreturn }
+attributes #4 = { nounwind }
+