[InstCombine] Improve strncpy to memcpy optimization

[xbolva00]

Summary

We optimize
int f1(void) {

char buf[20];
strncpy(buf, "hi", 3);
puts(buf);

}

to memcpy, but we bail out in the following case:

int f2(void) {

char buf[20];
strncpy(buf, "hi", 4);
puts(buf);

}

This patch fixes it.

Godbolt: https://godbolt.org/g/ganpSH

[xbolva00]

• Updated tests and description

[xbolva00]

"strncpy() writes additional null bytes to dest to ensure that a total of n bytes are written"

So we cant do this. (or we can if we know it the buf is zero initialized)

lib/Transforms/Utils/SimplifyLibCalls.cpp

Value *LibCallSimplifier::optimizeStrNCpy(CallInst *CI, IRBuilder<> &B) {
   if (ConstantInt *LengthArg = dyn_cast<ConstantInt>(LenOp))
     Len = LengthArg->getZExtValue();
   else
     return nullptr;
 
   if (Len == 0)
     return Dst; // strncpy(x, y, 0) -> x
 
-  // Let strncpy handle the zero padding
-  if (Len > SrcLen + 1)
-    return nullptr;
+  Len = std::min(Len, SrcLen + 1);
 
   Type *PT = Callee->getFunctionType()->getParamType(0);
   // strncpy(x, s, c) -> memcpy(align 1 x, align 1 s, c) [s and c are constant]
   B.CreateMemCpy(Dst, 1, Src, 1, ConstantInt::get(DL.getIntPtrType(PT), Len));
 
   return Dst;
 }
 

test/Transforms/InstCombine/strncpy-1.ll

+; NOTE: Assertions have been autogenerated by utils/update_test_checks.py
 ; Test that the strncpy library call simplifier works correctly.
 ;
 ; RUN: opt < %s -instcombine -S | FileCheck %s
 
 target datalayout = "e-p:32:32:32-i1:8:8-i8:8:8-i16:16:16-i32:32:32-i64:32:64-f32:32:32-f64:32:64-v64:64:64-v128:128:128-a0:0:64-f80:128:128"
 
 @hello = constant [6 x i8] c"hello\00"
 @null = constant [1 x i8] zeroinitializer
 @null_hello = constant [7 x i8] c"\00hello\00"
 @a = common global [32 x i8] zeroinitializer, align 1
 @b = common global [32 x i8] zeroinitializer, align 1
 
 declare i8* @strncpy(i8*, i8*, i32)
 declare i32 @puts(i8*)
 
 ; Check a bunch of strncpy invocations together.
 
 define i32 @test_simplify1() {
 ; CHECK-LABEL: @test_simplify1(
-; CHECK-NOT: call i8* @strncpy
-; CHECK: call i32 @puts
+; CHECK-NEXT:    [[TARGET:%.*]] = alloca [1024 x i8], align 1
+; CHECK-NEXT:    [[ARG1:%.*]] = getelementptr inbounds [1024 x i8], [1024 x i8]* [[TARGET]], i32 0, i32 0
+; CHECK-NEXT:    store i8 0, i8* [[ARG1]], align 1
+; CHECK-NEXT:    call void @llvm.memcpy.p0i8.p0i8.i32(i8* nonnull align 1 [[ARG1]], i8* align 1 getelementptr inbounds ([6 x i8], [6 x i8]* @hello, i32 0, i32 0), i32 6, i1 false)
+; CHECK-NEXT:    call void @llvm.memset.p0i8.i32(i8* nonnull align 1 [[ARG1]], i8 0, i32 42, i1 false)
+; CHECK-NEXT:    call void @llvm.memset.p0i8.i32(i8* nonnull align 1 [[ARG1]], i8 0, i32 42, i1 false)
+; CHECK-NEXT:    [[TMP1:%.*]] = call i32 @puts(i8* nonnull [[ARG1]])
+; CHECK-NEXT:    ret i32 0
+;
   %target = alloca [1024 x i8]
   %arg1 = getelementptr [1024 x i8], [1024 x i8]* %target, i32 0, i32 0
   store i8 0, i8* %arg1
 
   %arg2 = getelementptr [6 x i8], [6 x i8]* @hello, i32 0, i32 0
   %rslt1 = call i8* @strncpy(i8* %arg1, i8* %arg2, i32 6)
 
   %arg3 = getelementptr [1 x i8], [1 x i8]* @null, i32 0, i32 0
   %rslt2 = call i8* @strncpy(i8* %rslt1, i8* %arg3, i32 42)
 
   %arg4 = getelementptr [7 x i8], [7 x i8]* @null_hello, i32 0, i32 0
   %rslt3 = call i8* @strncpy(i8* %rslt2, i8* %arg4, i32 42)
 
   call i32 @puts( i8* %rslt3 )
   ret i32 0
 }
 
 ; Check strncpy(x, "", y) -> memset(x, '\0', y, 1).
 
 define void @test_simplify2() {
 ; CHECK-LABEL: @test_simplify2(
+; CHECK-NEXT:    call void @llvm.memset.p0i8.i32(i8* align 1 getelementptr inbounds ([32 x i8], [32 x i8]* @a, i32 0, i32 0), i8 0, i32 32, i1 false)
+; CHECK-NEXT:    ret void
+;
   %dst = getelementptr [32 x i8], [32 x i8]* @a, i32 0, i32 0
   %src = getelementptr [1 x i8], [1 x i8]* @null, i32 0, i32 0
 
   call i8* @strncpy(i8* %dst, i8* %src, i32 32)
-; CHECK: call void @llvm.memset.p0i8.i32
   ret void
 }
 
 ; Check strncpy(x, y, 0) -> x.
 
 define i8* @test_simplify3() {
 ; CHECK-LABEL: @test_simplify3(
+; CHECK-NEXT:    ret i8* getelementptr inbounds ([32 x i8], [32 x i8]* @a, i32 0, i32 0)
+;
   %dst = getelementptr [32 x i8], [32 x i8]* @a, i32 0, i32 0
   %src = getelementptr [6 x i8], [6 x i8]* @hello, i32 0, i32 0
 
   %ret = call i8* @strncpy(i8* %dst, i8* %src, i32 0)
   ret i8* %ret
-; CHECK: ret i8* getelementptr inbounds ([32 x i8], [32 x i8]* @a, i32 0, i32 0)
 }
 
 ; Check  strncpy(x, s, c) -> memcpy(x, s, c, 1) [s and c are constant].
 
 define void @test_simplify4() {
 ; CHECK-LABEL: @test_simplify4(
+; CHECK-NEXT:    call void @llvm.memcpy.p0i8.p0i8.i32(i8* align 1 getelementptr inbounds ([32 x i8], [32 x i8]* @a, i32 0, i32 0), i8* align 1 getelementptr inbounds ([6 x i8], [6 x i8]* @hello, i32 0, i32 0), i32 6, i1 false)
+; CHECK-NEXT:    ret void
+;
   %dst = getelementptr [32 x i8], [32 x i8]* @a, i32 0, i32 0
   %src = getelementptr [6 x i8], [6 x i8]* @hello, i32 0, i32 0
 
   call i8* @strncpy(i8* %dst, i8* %src, i32 6)
-; CHECK: call void @llvm.memcpy.p0i8.p0i8.i32
+  ret void
+}
+
+; Check  strncpy(x, s, c) -> memcpy(x, s, min(c, len(s)), 1) [s and c are constant].
+define void @test_simplify5() {
+; CHECK-LABEL: @test_no_simplify2(
+; CHECK-NEXT:    call void @llvm.memcpy.p0i8.p0i8.i32(i8* align 1 getelementptr inbounds ([32 x i8], [32 x i8]* @a, i32 0, i32 0), i8* align 1 getelementptr inbounds ([6 x i8], [6 x i8]* @hello, i32 0, i32 0), i32 6, i1 false)
+; CHECK-NEXT:    ret void
+;
+  %dst = getelementptr [32 x i8], [32 x i8]* @a, i32 0, i32 0
+  %src = getelementptr [6 x i8], [6 x i8]* @hello, i32 0, i32 0
+
+  call i8* @strncpy(i8* %dst, i8* %src, i32 8)
   ret void
 }
 
 ; Check cases that shouldn't be simplified.
 
 define void @test_no_simplify1() {
 ; CHECK-LABEL: @test_no_simplify1(
+; CHECK-NEXT:    [[TMP1:%.*]] = call i8* @strncpy(i8* getelementptr inbounds ([32 x i8], [32 x i8]* @a, i32 0, i32 0), i8* getelementptr inbounds ([32 x i8], [32 x i8]* @b, i32 0, i32 0), i32 32)
+; CHECK-NEXT:    ret void
+;
   %dst = getelementptr [32 x i8], [32 x i8]* @a, i32 0, i32 0
   %src = getelementptr [32 x i8], [32 x i8]* @b, i32 0, i32 0
 
   call i8* @strncpy(i8* %dst, i8* %src, i32 32)
-; CHECK: call i8* @strncpy
-  ret void
-}
-
-define void @test_no_simplify2() {
-; CHECK-LABEL: @test_no_simplify2(
-  %dst = getelementptr [32 x i8], [32 x i8]* @a, i32 0, i32 0
-  %src = getelementptr [6 x i8], [6 x i8]* @hello, i32 0, i32 0
-
-  call i8* @strncpy(i8* %dst, i8* %src, i32 8)
-; CHECK: call i8* @strncpy
   ret void
 }