GVN: Enable value forwarding for calloc

Enable value forwarding for loads from `calloc()` without an intervening
store.

This change extends GVN to handle the following case:

    %1 = tail call noalias i8* @calloc(i64 1, i64 4)
    %2 = bitcast i8* %1 to i32*
    ; This load is trivially constant zero
    %3 = load i32* %2, align 4

This is analogous to the handling for `malloc()` in the same places.
`malloc()` returns `undef`; `calloc()` returns a zero value.  Note that
it is correct to return zero even for out of bounds GEPs since the
result of such a GEP would be undefined.

Patch by Philip Reames!

llvm-svn: 210828

2 files changed, 41 insertions, 0 deletions

diff --git a/llvm/lib/Transforms/Scalar/GVN.cpp b/llvm/lib/Transforms/Scalar/GVN.cpp
index 6d07dddd3e5..56781d44aaa 100644
--- a/llvm/lib/Transforms/Scalar/GVN.cpp
+++ b/llvm/lib/Transforms/Scalar/GVN.cpp
@@ -1464,6 +1464,13 @@ void GVN::AnalyzeLoadAvailability(LoadInst *LI, LoadDepVect &Deps,
       continue;
     }
 
+    // Loading from calloc (which zero initializes memory) -> zero
+    if (isCallocLikeFn(DepInst, TLI)) {
+      ValuesPerBlock.push_back(AvailableValueInBlock::get(
+          DepBB, Constant::getNullValue(LI->getType())));
+      continue;
+    }
+
     if (StoreInst *S = dyn_cast<StoreInst>(DepInst)) {
       // Reject loads and stores that are to the same address but are of
       // different types if we have to.
@@ -1988,6 +1995,15 @@ bool GVN::processLoad(LoadInst *L) {
     }
   }
 
+  // If this load follows a calloc (which zero initializes memory),
+  // then the loaded value is zero
+  if (isCallocLikeFn(DepInst, TLI)) {
+    L->replaceAllUsesWith(Constant::getNullValue(L->getType()));
+    markInstructionForDeletion(L);
+    ++NumGVNLoad;
+    return true;
+  }
+
   return false;
 }
 
diff --git a/llvm/test/Transforms/GVN/calloc-load-removal.ll b/llvm/test/Transforms/GVN/calloc-load-removal.ll
new file mode 100644
index 00000000000..2dde5b7b414
--- /dev/null
+++ b/llvm/test/Transforms/GVN/calloc-load-removal.ll
@@ -0,0 +1,25 @@
+; RUN: opt -S -basicaa -gvn < %s | FileCheck %s
+; RUN: opt -S -basicaa -gvn -disable-simplify-libcalls < %s | FileCheck %s -check-prefix=CHECK_NO_LIBCALLS
+; Check that loads from calloc are recognized as being zero.
+
+target datalayout = "e-p:64:64:64-i1:8:8-i8:8:8-i16:16:16-i32:32:32-i64:64:64-f32:32:32-f64:64:64-v64:64:64-v128:128:128-a0:0:64-s0:64:64-f80:128:128-n8:16:32:64-S128"
+
+; Function Attrs: nounwind uwtable
+define i32 @test1() {
+  %1 = tail call noalias i8* @calloc(i64 1, i64 4)
+  %2 = bitcast i8* %1 to i32*
+  ; This load is trivially constant zero
+  %3 = load i32* %2, align 4
+  ret i32 %3
+
+; CHECK-LABEL: @test1(
+; CHECK-NOT: %3 = load i32* %2, align 4
+; CHECK: ret i32 0
+
+; CHECK_NO_LIBCALLS-LABEL: @test1(
+; CHECK_NO_LIBCALLS: load
+; CHECK_NO_LIBCALLS: ret i32 %
+
+}
+
+declare noalias i8* @calloc(i64, i64)