Index: lib/LTO/UpdateCompilerUsed.cpp
===================================================================
--- lib/LTO/UpdateCompilerUsed.cpp
+++ lib/LTO/UpdateCompilerUsed.cpp
@@ -95,12 +95,18 @@
     if (GV.hasPrivateLinkage())
       return;
 
-    // Conservatively append user-supplied runtime library functions to
-    // llvm.compiler.used.  These could be internalized and deleted by
-    // optimizations like -globalopt, causing problems when later optimizations
-    // add new library calls (e.g., llvm.memset => memset and printf => puts).
+    // Conservatively append user-supplied runtime library functions (supplied
+    // either directly, or via a function alias) to llvm.compiler.used.  These
+    // could be internalized and deleted by optimizations like -globalopt,
+    // causing problems when later optimizations add new library calls (e.g.,
+    // llvm.memset => memset and printf => puts).
     // Leave it to the linker to remove any dead code (e.g. with -dead_strip).
-    if (isa<Function>(GV) && Libcalls.count(GV.getName())) {
+    GlobalValue *FuncAliasee = nullptr;
+    if (isa<GlobalAlias>(GV)) {
+      auto *A = cast<GlobalAlias>(&GV);
+      FuncAliasee = dyn_cast<Function>(A->getAliasee());
+    }
+    if ((isa<Function>(GV) || FuncAliasee) && Libcalls.count(GV.getName())) {
       LLVMUsed.push_back(&GV);
       return;
     }
Index: test/LTO/X86/libcall-overridden-via-alias.ll
===================================================================
--- test/LTO/X86/libcall-overridden-via-alias.ll
+++ test/LTO/X86/libcall-overridden-via-alias.ll
@@ -0,0 +1,69 @@
+; Given a library call that is represented as an llvm intrinsic call, but
+; later transformed to an actual call, if an overriding definition of that
+; library routine is provided indirectly via an alias, verify that LTO
+; does not eliminate the definition.  This is a test for PR38547.
+;
+; RUN: llvm-as -o %t1 %s
+; RUN: llvm-lto -exported-symbol=main -save-merged-module -filetype=asm -o %t2 %t1
+; RUN: llvm-dis -o - %t2.merged.bc | FileCheck --check-prefix=CHECK_IR %s
+;
+; Check that the call is represented as an llvm intrinsic in the IR after LTO:
+; CHECK_IR-LABEL: main
+; CHECK_IR: call float @llvm.log.f32
+;
+; Check that the IR contains the overriding definition of the library routine
+; in the IR after LTO:
+; CHECK_IR: define internal float @logf(float [[X:%.*]])
+; CHECK_IR-NEXT:   [[TMP:%.*]] = fadd float [[X]], [[X]]
+; CHECK_IR-NEXT:   ret float [[TMP]]
+;
+; Check that the assembly code from LTO contains the call to the expected
+; library routine, and that the overriding definition of the library routine
+; is present:
+; RUN: FileCheck --check-prefix=CHECK_ASM %s < %t2
+; CHECK_ASM-LABEL: main:
+; CHECK_ASM: callq logf
+; CHECK_ASM-LABEL: logf:
+; CHECK_ASM-NEXT: add
+; CHECK_ASM-NEXT: ret
+
+; Produced from the following source-code:
+;
+;extern float logf(float);
+;// 'src' and 'dst' are 'volatile' to prohibit optimization.
+;volatile float src = 3.14f;
+;volatile float dst;
+;
+;int main() {
+;  dst = logf(src);
+;  return 0;
+;}
+;
+;extern float fname(float x);
+;float fname(float x) {
+;  return x + x;
+;}
+;
+;float logf(float x) __attribute__((alias("fname")));
+;
+target triple = "x86_64-unknown-linux-gnu"
+
+@src = global float 0x40091EB860000000, align 4
+@dst = common global float 0.000000e+00, align 4
+
+@logf = alias float (float), float (float)* @fname
+
+define i32 @main() local_unnamed_addr {
+entry:
+  %0 = load volatile float, float* @src, align 4
+  %1 = tail call float @llvm.log.f32(float %0)
+  store volatile float %1, float* @dst, align 4
+  ret i32 0
+}
+
+declare float @llvm.log.f32(float)
+
+define float @fname(float %x) {
+  %add = fadd float %x, %x
+  ret float %add
+}