Index: llvm/lib/Transforms/Utils/SimplifyLibCalls.cpp
===================================================================
--- llvm/lib/Transforms/Utils/SimplifyLibCalls.cpp
+++ llvm/lib/Transforms/Utils/SimplifyLibCalls.cpp
@@ -2591,6 +2591,14 @@
     }
 
     if (FormatStr[1] == 's') {
+      // snprintf(NULL, 0, "%s", str) to strlen(str)
+      if (isa<ConstantPointerNull>(CI->getArgOperand(0)) && N == 0)
+        if (auto *V = emitStrLen(CI->getArgOperand(3), B, DL, TLI)) {
+          if (CI->getType() != V->getType())
+            V = B.CreateTrunc(V, CI->getType());
+          return V;
+        }
+
       // snprintf(dest, size, "%s", str) to llvm.memcpy(dest, str, len+1, 1)
       StringRef Str;
       if (!getConstantStringInfo(CI->getArgOperand(3), Str))
Index: llvm/test/Transforms/InstCombine/snprintf.ll
===================================================================
--- llvm/test/Transforms/InstCombine/snprintf.ll
+++ llvm/test/Transforms/InstCombine/snprintf.ll
@@ -136,3 +136,23 @@
   %call = call i32 (i8*, i64, i8*, ...) @snprintf(i8* %buf, i64 32, i8* getelementptr inbounds ([3 x i8], [3 x i8]* @.str.3, i64 0, i64 0), i8* getelementptr inbounds ([4 x i8], [4 x i8]* @.str, i64 0, i64 0)) #2
   ret i32 %call
 }
+
+
+define i32 @snprint_unknown_str_len(i8* %fmt, i8* %s) {
+; CHECK-LABEL: @snprint_unknown_str_len(
+; CHECK-NEXT:    [[LEN:%.*]] = call i32 (i8*, i64, i8*, ...) @snprintf(i8* null, i64 0, i8* [[FMT:%.*]], i8* [[S:%.*]])
+; CHECK-NEXT:    ret i32 [[LEN]]
+;
+  %len = call i32 (i8*, i64, i8*, ...) @snprintf(i8* null, i64 0, i8* %fmt, i8* %s)
+  ret i32 %len
+}
+
+define i32 @snprint_to_strlen(i8* %s) {
+; CHECK-LABEL: @snprint_to_strlen(
+; CHECK-NEXT:    [[STRLEN:%.*]] = call i64 @strlen(i8* noundef nonnull dereferenceable(1) [[S:%.*]])
+; CHECK-NEXT:    [[TMP1:%.*]] = trunc i64 [[STRLEN]] to i32
+; CHECK-NEXT:    ret i32 [[TMP1]]
+;
+  %len = call i32 (i8*, i64, i8*, ...) @snprintf(i8* null, i64 0, i8* getelementptr inbounds ([3 x i8], [3 x i8]* @.str.3, i64 0, i64 0), i8* %s)
+  ret i32 %len
+}