[llvm] 5ef2287 - [SLC] Optimize strncpy(a, a, C) to memcpy(a, a000, C)

[Eli Friedman via llvm-commits]

We probably want to limit the number of null characters we append here; if the length is long, we could generate an excessively long global.

The strncpy call in fill_with_zeros2 has undefined behavior; is that intentional?

-Eli

-----Original Message-----
From: llvm-commits <llvm-commits-bounces at lists.llvm.org> On Behalf Of Dávid Bolvanský via llvm-commits
Sent: Thursday, August 13, 2020 1:23 PM
To: llvm-commits at lists.llvm.org
Subject: [EXT] [llvm] 5ef2287 - [SLC] Optimize strncpy(a, a, C) to memcpy(a, a000, C)


Author: Dávid Bolvanský
Date: 2020-08-13T22:22:51+02:00
New Revision: 5ef2287d36f93bac3b7b8c49c1b371d25d305e85

URL: https://github.com/llvm/llvm-project/commit/5ef2287d36f93bac3b7b8c49c1b371d25d305e85
DIFF: https://github.com/llvm/llvm-project/commit/5ef2287d36f93bac3b7b8c49c1b371d25d305e85.diff

LOG: [SLC] Optimize strncpy(a, a, C) to memcpy(a, a000, C)
Solves PR47154

Added:
    llvm/test/Transforms/InstCombine/strncpy-3.ll

Modified:
    llvm/lib/Transforms/Utils/SimplifyLibCalls.cpp

Removed:



################################################################################
diff  --git a/llvm/lib/Transforms/Utils/SimplifyLibCalls.cpp b/llvm/lib/Transforms/Utils/SimplifyLibCalls.cpp
index b03389aec7bf..2f6e60fc09c4 100644
--- a/llvm/lib/Transforms/Utils/SimplifyLibCalls.cpp
+++ b/llvm/lib/Transforms/Utils/SimplifyLibCalls.cpp
@@ -608,9 +608,15 @@ Value *LibCallSimplifier::optimizeStrNCpy(CallInst *CI, IRBuilderBase &B) {
     return Dst;
   }

-  // Let strncpy handle the zero padding
-  if (Len > SrcLen + 1)
-    return nullptr;
+  // strncpy(a, "a", 4) - > memcpy(a, "a\0\0\0", 4)
+  if (Len > SrcLen + 1) {
+    StringRef Str;
+    if (!getConstantStringInfo(Src, Str))
+      return nullptr;
+    std::string SrcStr = Str.str();
+    SrcStr.resize(Len, '\0');
+    Src = B.CreateGlobalString(SrcStr, "str");
+  }

   Type *PT = Callee->getFunctionType()->getParamType(0);
   // strncpy(x, s, c) -> memcpy(align 1 x, align 1 s, c) [s and c are constant]

diff  --git a/llvm/test/Transforms/InstCombine/strncpy-3.ll b/llvm/test/Transforms/InstCombine/strncpy-3.ll
new file mode 100644
index 000000000000..744f1e4169e0
--- /dev/null
+++ b/llvm/test/Transforms/InstCombine/strncpy-3.ll
@@ -0,0 +1,40 @@
+; NOTE: Assertions have been autogenerated by utils/update_test_checks.py
+; RUN: opt < %s -instcombine -S | FileCheck %s
+
+
+ at str = constant [2 x i8] c"a\00"
+ at str2 = constant [3 x i8] c"abc"
+ at str3 = constant [4 x i8] c"abcd"
+
+declare i8* @strncpy(i8*, i8*, i64)
+
+
+define void @fill_with_zeros(i8* %dst) {
+; CHECK-LABEL: @fill_with_zeros(
+; CHECK-NEXT:    [[TMP1:%.*]] = bitcast i8* [[DST:%.*]] to i32*
+; CHECK-NEXT:    store i32 97, i32* [[TMP1]], align 1
+; CHECK-NEXT:    ret void
+;
+  tail call i8* @strncpy(i8* %dst, i8* getelementptr inbounds ([2 x i8], [2 x i8]* @str, i64 0, i64 0), i64 4)
+  ret void
+}
+
+define void @fill_with_zeros2(i8* %dst) {
+; CHECK-LABEL: @fill_with_zeros2(
+; CHECK-NEXT:    [[TMP1:%.*]] = bitcast i8* [[DST:%.*]] to i32*
+; CHECK-NEXT:    store i32 6513249, i32* [[TMP1]], align 1
+; CHECK-NEXT:    ret void
+;
+  tail call i8* @strncpy(i8* %dst, i8* getelementptr inbounds ([3 x i8], [3 x i8]* @str2, i64 0, i64 0), i64 4)
+  ret void
+}
+
+define void @fill_with_zeros3(i8* %dst) {
+; CHECK-LABEL: @fill_with_zeros3(
+; CHECK-NEXT:    [[TMP1:%.*]] = bitcast i8* [[DST:%.*]] to i32*
+; CHECK-NEXT:    store i32 1684234849, i32* [[TMP1]], align 1
+; CHECK-NEXT:    ret void
+;
+  tail call i8* @strncpy(i8* %dst, i8* getelementptr inbounds ([4 x i8], [4 x i8]* @str3, i64 0, i64 0), i64 4)
+  ret void
+}



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