Index: lib/CodeGen/SelectionDAG/SelectionDAGBuilder.cpp
===================================================================
--- lib/CodeGen/SelectionDAG/SelectionDAGBuilder.cpp
+++ lib/CodeGen/SelectionDAG/SelectionDAGBuilder.cpp
@@ -6207,9 +6207,10 @@
     }
 
     // Check for well-known libc/libm calls.  If the function is internal, it
-    // can't be a library call.
+    // can't be a library call.  Don't do the check if marked as nobuiltin for
+    // some reason.
     LibFunc::Func Func;
-    if (!F->hasLocalLinkage() && F->hasName() &&
+    if (!I.isNoBuiltin() && !F->hasLocalLinkage() && F->hasName() &&
         LibInfo->getLibFunc(F->getName(), Func) &&
         LibInfo->hasOptimizedCodeGen(Func)) {
       switch (Func) {
Index: test/CodeGen/SystemZ/memchr-nobuiltin.ll
===================================================================
--- /dev/null
+++ test/CodeGen/SystemZ/memchr-nobuiltin.ll
@@ -0,0 +1,16 @@
+; Test that memchr won't be converted to SRST if calls are
+; marked with nobuiltin, eg. for sanitizers.
+;
+; RUN: llc < %s -mtriple=s390x-linux-gnu | FileCheck %s
+
+declare i8 *@memchr(i8 *%src, i16 %char, i32 %len)
+
+; Test a simple forwarded call.
+define i8 *@f1(i8 *%src, i16 %char, i32 %len) {
+; CHECK-LABEL: f1:
+; CHECK-NOT: srst
+; CHECK: brasl %r14, memchr
+; CHECK: br %r14
+  %res = call i8 *@memchr(i8 *%src, i16 %char, i32 %len) nobuiltin
+  ret i8 *%res
+}
Index: test/CodeGen/SystemZ/memcmp-nobuiltin.ll
===================================================================
--- /dev/null
+++ test/CodeGen/SystemZ/memcmp-nobuiltin.ll
@@ -0,0 +1,191 @@
+; Test that memcmp won't be converted to CLC if calls are
+; marked with nobuiltin, eg. for sanitizers.
+;
+; RUN: llc < %s -mtriple=s390x-linux-gnu | FileCheck %s
+
+declare signext i32 @memcmp(i8 *%src1, i8 *%src2, i64 %size)
+
+; Zero-length comparisons should be optimized away.
+define i32 @f1(i8 *%src1, i8 *%src2) {
+; CHECK-LABEL: f1:
+; CHECK-NOT: clc
+; CHECK: brasl %r14, memcmp
+; CHECK: br %r14
+  %res = call i32 @memcmp(i8 *%src1, i8 *%src2, i64 0) nobuiltin
+  ret i32 %res
+}
+
+; Check a case where the result is used as an integer.
+define i32 @f2(i8 *%src1, i8 *%src2) {
+; CHECK-LABEL: f2:
+; CHECK-NOT: clc
+; CHECK: brasl %r14, memcmp
+; CHECK: br %r14
+  %res = call i32 @memcmp(i8 *%src1, i8 *%src2, i64 2) nobuiltin
+  ret i32 %res
+}
+
+; Check a case where the result is tested for equality.
+define void @f3(i8 *%src1, i8 *%src2, i32 *%dest) {
+; CHECK-LABEL: f3:
+; CHECK-NOT: clc
+; CHECK: brasl %r14, memcmp
+; CHECK: br %r14
+  %res = call i32 @memcmp(i8 *%src1, i8 *%src2, i64 3) nobuiltin
+  %cmp = icmp eq i32 %res, 0
+  br i1 %cmp, label %exit, label %store
+
+store:
+  store i32 0, i32 *%dest
+  br label %exit
+
+exit:
+  ret void
+}
+
+; Check a case where the result is tested for inequality.
+define void @f4(i8 *%src1, i8 *%src2, i32 *%dest) {
+; CHECK-LABEL: f4:
+; CHECK-NOT: clc
+; CHECK: brasl %r14, memcmp
+; CHECK: br %r14
+entry:
+  %res = call i32 @memcmp(i8 *%src1, i8 *%src2, i64 4) nobuiltin
+  %cmp = icmp ne i32 %res, 0
+  br i1 %cmp, label %exit, label %store
+
+store:
+  store i32 0, i32 *%dest
+  br label %exit
+
+exit:
+  ret void
+}
+
+; Check a case where the result is tested via slt.
+define void @f5(i8 *%src1, i8 *%src2, i32 *%dest) {
+; CHECK-LABEL: f5:
+; CHECK-NOT: clc
+; CHECK: brasl %r14, memcmp
+; CHECK: br %r14
+entry:
+  %res = call i32 @memcmp(i8 *%src1, i8 *%src2, i64 5) nobuiltin
+  %cmp = icmp slt i32 %res, 0
+  br i1 %cmp, label %exit, label %store
+
+store:
+  store i32 0, i32 *%dest
+  br label %exit
+
+exit:
+  ret void
+}
+
+; Check a case where the result is tested for sgt.
+define void @f6(i8 *%src1, i8 *%src2, i32 *%dest) {
+; CHECK-LABEL: f6:
+; CHECK-NOT: clc
+; CHECK: brasl %r14, memcmp
+; CHECK: br %r14
+entry:
+  %res = call i32 @memcmp(i8 *%src1, i8 *%src2, i64 6) nobuiltin
+  %cmp = icmp sgt i32 %res, 0
+  br i1 %cmp, label %exit, label %store
+
+store:
+  store i32 0, i32 *%dest
+  br label %exit
+
+exit:
+  ret void
+}
+
+; Check the upper end of the CLC range.  Here the result is used both as
+; an integer and for branching.
+define i32 @f7(i8 *%src1, i8 *%src2, i32 *%dest) {
+; CHECK-LABEL: f7:
+; CHECK-NOT: clc
+; CHECK: brasl %r14, memcmp
+; CHECK: br %r14
+entry:
+  %res = call i32 @memcmp(i8 *%src1, i8 *%src2, i64 256) nobuiltin
+  %cmp = icmp slt i32 %res, 0
+  br i1 %cmp, label %exit, label %store
+
+store:
+  store i32 0, i32 *%dest
+  br label %exit
+
+exit:
+  ret i32 %res
+}
+
+; 257 bytes needs two CLCs.
+define i32 @f8(i8 *%src1, i8 *%src2) {
+; CHECK-LABEL: f8:
+; CHECK-NOT: clc
+; CHECK: brasl %r14, memcmp
+; CHECK: br %r14
+  %res = call i32 @memcmp(i8 *%src1, i8 *%src2, i64 257) nobuiltin
+  ret i32 %res
+}
+
+; Test a comparison of 258 bytes in which the CC result can be used directly.
+define void @f9(i8 *%src1, i8 *%src2, i32 *%dest) {
+; CHECK-LABEL: f9:
+; CHECK-NOT: clc
+; CHECK: brasl %r14, memcmp
+; CHECK: br %r14
+entry:
+  %res = call i32 @memcmp(i8 *%src1, i8 *%src2, i64 257) nobuiltin
+  %cmp = icmp slt i32 %res, 0
+  br i1 %cmp, label %exit, label %store
+
+store:
+  store i32 0, i32 *%dest
+  br label %exit
+
+exit:
+  ret void
+}
+
+; Test the largest size that can use two CLCs.
+define i32 @f10(i8 *%src1, i8 *%src2) {
+; CHECK-LABEL: f10:
+; CHECK-NOT: clc
+; CHECK: brasl %r14, memcmp
+; CHECK: br %r14
+  %res = call i32 @memcmp(i8 *%src1, i8 *%src2, i64 512) nobuiltin
+  ret i32 %res
+}
+
+; Test the smallest size that needs 3 CLCs.
+define i32 @f11(i8 *%src1, i8 *%src2) {
+; CHECK-LABEL: f11:
+; CHECK-NOT: clc
+; CHECK: brasl %r14, memcmp
+; CHECK: br %r14
+  %res = call i32 @memcmp(i8 *%src1, i8 *%src2, i64 513) nobuiltin
+  ret i32 %res
+}
+
+; Test the largest size than can use 3 CLCs.
+define i32 @f12(i8 *%src1, i8 *%src2) {
+; CHECK-LABEL: f12:
+; CHECK-NOT: clc
+; CHECK: brasl %r14, memcmp
+; CHECK: br %r14
+  %res = call i32 @memcmp(i8 *%src1, i8 *%src2, i64 768) nobuiltin
+  ret i32 %res
+}
+
+; The next size up uses a loop instead.  We leave the more complicated
+; loop tests to memcpy-01.ll, which shares the same form.
+define i32 @f13(i8 *%src1, i8 *%src2) {
+; CHECK-LABEL: f13:
+; CHECK-NOT: clc
+; CHECK: brasl %r14, memcmp
+; CHECK: br %r14
+  %res = call i32 @memcmp(i8 *%src1, i8 *%src2, i64 769) nobuiltin
+  ret i32 %res
+}
Index: test/CodeGen/SystemZ/strcmp-nobuiltin.ll
===================================================================
--- /dev/null
+++ test/CodeGen/SystemZ/strcmp-nobuiltin.ll
@@ -0,0 +1,54 @@
+; Test that strcmp won't be converted to CLST if calls are
+; marked with nobuiltin, eg. for sanitizers.
+;
+; RUN: llc < %s -mtriple=s390x-linux-gnu | FileCheck %s
+
+declare signext i32 @strcmp(i8 *%src1, i8 *%src2)
+
+; Check a case where the result is used as an integer.
+define i32 @f1(i8 *%src1, i8 *%src2) {
+; CHECK-LABEL: f1:
+; CHECK-NOT: clst
+; CHECK: brasl %r14, strcmp
+; CHECK: br %r14
+  %res = call i32 @strcmp(i8 *%src1, i8 *%src2) nobuiltin
+  ret i32 %res
+}
+
+; Check a case where the result is tested for equality.
+define void @f2(i8 *%src1, i8 *%src2, i32 *%dest) {
+; CHECK-LABEL: f2:
+; CHECK-NOT: clst
+; CHECK: brasl %r14, strcmp
+; CHECK: br %r14
+  %res = call i32 @strcmp(i8 *%src1, i8 *%src2) nobuiltin
+  %cmp = icmp eq i32 %res, 0
+  br i1 %cmp, label %exit, label %store
+
+store:
+  store i32 0, i32 *%dest
+  br label %exit
+
+exit:
+  ret void
+}
+
+; Test a case where the result is used both as an integer and for
+; branching.
+define i32 @f3(i8 *%src1, i8 *%src2, i32 *%dest) {
+; CHECK-LABEL: f3:
+; CHECK-NOT: clst
+; CHECK: brasl %r14, strcmp
+; CHECK: br %r14
+entry:
+  %res = call i32 @strcmp(i8 *%src1, i8 *%src2) nobuiltin
+  %cmp = icmp slt i32 %res, 0
+  br i1 %cmp, label %exit, label %store
+
+store:
+  store i32 0, i32 *%dest
+  br label %exit
+
+exit:
+  ret i32 %res
+}
Index: test/CodeGen/SystemZ/strcpy-nobuiltin.ll
===================================================================
--- /dev/null
+++ test/CodeGen/SystemZ/strcpy-nobuiltin.ll
@@ -0,0 +1,42 @@
+; Test that strcmp won't be converted to MVST if calls are
+; marked with nobuiltin, eg. for sanitizers.
+;
+; RUN: llc < %s -mtriple=s390x-linux-gnu | FileCheck %s
+
+declare i8 *@strcpy(i8 *%dest, i8 *%src)
+declare i8 *@stpcpy(i8 *%dest, i8 *%src)
+
+; Check strcpy.
+define i8 *@f1(i8 *%dest, i8 *%src) {
+; CHECK-LABEL: f1:
+; CHECK-NOT: mvst
+; CHECK: brasl %r14, strcpy
+; CHECK: br %r14
+  %res = call i8 *@strcpy(i8 *%dest, i8 *%src) nobuiltin
+  ret i8 *%res
+}
+
+; Check stpcpy.
+define i8 *@f2(i8 *%dest, i8 *%src) {
+; CHECK-LABEL: f2:
+; CHECK-NOT: mvst
+; CHECK: brasl %r14, stpcpy
+; CHECK: br %r14
+  %res = call i8 *@stpcpy(i8 *%dest, i8 *%src) nobuiltin
+  ret i8 *%res
+}
+
+; Check correct operation with other loads and stores.  The load must
+; come before the loop and the store afterwards.
+define i32 @f3(i32 %dummy, i8 *%dest, i8 *%src, i32 *%resptr, i32 *%storeptr) {
+; CHECK-LABEL: f3:
+; CHECK-DAG: l [[REG1:%r[0-9]+]], 0(%r5)
+; CHECK-NOT: mvst
+; CHECK: brasl %r14, strcpy
+; CHECK: mvhi 0(%r6), 0
+; CHECK: br %r14
+  %res = load i32 , i32 *%resptr
+  %unused = call i8 *@strcpy(i8 *%dest, i8 *%src) nobuiltin
+  store i32 0, i32 *%storeptr
+  ret i32 %res
+}
Index: test/CodeGen/SystemZ/strlen-nobuiltin.ll
===================================================================
--- /dev/null
+++ test/CodeGen/SystemZ/strlen-nobuiltin.ll
@@ -0,0 +1,25 @@
+; Test that strlen/strnlen won't be converted to SRST if calls are
+; marked with nobuiltin, eg. for sanitizers.
+;
+; RUN: llc < %s -mtriple=s390x-linux-gnu | FileCheck %s
+
+declare i64 @strlen(i8 *%src)
+declare i64 @strnlen(i8 *%src, i64 %len)
+
+define i64 @f1(i32 %dummy, i8 *%src) {
+; CHECK-LABEL: f1:
+; CHECK: brasl %r14, strlen
+; CHECK: br %r14
+  %res = call i64 @strlen(i8 *%src) nobuiltin
+  ret i64 %res
+}
+
+; Likewise for strnlen.
+define i64 @f2(i64 %len, i8 *%src) {
+; CHECK-LABEL: f2:
+; CHECK-NOT: srst
+; CHECK: brasl %r14, strnlen
+; CHECK: br %r14
+  %res = call i64 @strnlen(i8 *%src, i64 %len) nobuiltin
+  ret i64 %res
+}
Index: test/CodeGen/X86/memcmp.ll
===================================================================
--- test/CodeGen/X86/memcmp.ll
+++ test/CodeGen/X86/memcmp.ll
@@ -45,6 +45,21 @@
 ; CHECK-NEXT: cmpl    $28527,
 }
 
+define void @memcmp2nb(i8* %X, i8* %Y, i32* nocapture %P) nounwind {
+entry:
+  %0 = tail call i32 (...) @memcmp(i8* %X, i8* %Y, i32 2) nounwind nobuiltin ; <i32> [#uses=1]
+  %1 = icmp eq i32 %0, 0                          ; <i1> [#uses=1]
+  br i1 %1, label %return, label %bb
+
+bb:                                               ; preds = %entry
+  store i32 4, i32* %P, align 4
+  ret void
+
+return:                                           ; preds = %entry
+  ret void
+; CHECK-LABEL: memcmp2nb:
+; CHECK: callq
+}
 
 define void @memcmp4(i8* %X, i8* %Y, i32* nocapture %P) nounwind {
 entry: