[LTO] Fix assertion failed when flushing bitcode incrementally for LTO output.

In https://reviews.llvm.org/D86905, we introduce an optimization, when lld emits LLVM bitcode,
we allow bitcode writer flush data to disk early when buffered data size is above some threshold.

But when `--plugin-opt=emit-llvm` and `-o /dev/null` are used,
lld will trigger assertion `BytesRead >= 0 && static_cast<size_t>(BytesRead) == BytesFromDisk`.
When we write output to /dev/null, BytesRead is zero, but at this program point BytesFromDisk is always non-zero.

Reviewed By: stephan.yichao.zhao, MaskRay

Differential Revision: https://reviews.llvm.org/D112297

diff --git a/lld/test/ELF/lto/emit-llvm.ll b/lld/test/ELF/lto/emit-llvm.ll
index a2020b4..e80ef57 100644 (file)
--- a/lld/test/ELF/lto/emit-llvm.ll
+++ b/lld/test/ELF/lto/emit-llvm.ll
@@ -4,6 +4,10 @@
 ; RUN: ld.lld --plugin-opt=emit-llvm -o %t.out.o %t.o
 ; RUN: llvm-dis < %t.out.o -o - | FileCheck %s
 
+;; Regression test for D112297: bitcode writer used to crash when
+;; --plugin-opt=emit-llvmis enabled and the output is /dev/null.
+; RUN: ld.lld --plugin-opt=emit-llvm -mllvm -bitcode-flush-threshold=0 -o /dev/null %t.o
+
 ; CHECK: define internal void @main()
 
 target datalayout = "e-m:e-p270:32:32-p271:32:32-p272:64:64-i64:64-f80:128-n8:16:32:64-S128"

diff --git a/llvm/include/llvm/Support/raw_ostream.h b/llvm/include/llvm/Support/raw_ostream.h
index 98c26ef..995b660 100644 (file)
--- a/llvm/include/llvm/Support/raw_ostream.h
+++ b/llvm/include/llvm/Support/raw_ostream.h
@@ -444,6 +444,7 @@ class raw_fd_ostream : public raw_pwrite_stream {
   int FD;
   bool ShouldClose;
   bool SupportsSeeking = false;
+  bool IsRegularFile = false;
   mutable Optional<bool> HasColors;
 
 #ifdef _WIN32
@@ -514,6 +515,8 @@ public:
 
   bool supportsSeeking() const { return SupportsSeeking; }
 
+  bool isRegularFile() const { return IsRegularFile; }
+
   /// Flushes the stream and repositions the underlying file descriptor position
   /// to the offset specified from the beginning of the file.
   uint64_t seek(uint64_t off);

diff --git a/llvm/lib/Support/raw_ostream.cpp b/llvm/lib/Support/raw_ostream.cpp
index 4590a3d..6881c7b 100644 (file)
--- a/llvm/lib/Support/raw_ostream.cpp
+++ b/llvm/lib/Support/raw_ostream.cpp
@@ -643,13 +643,14 @@ raw_fd_ostream::raw_fd_ostream(int fd, bool shouldClose, bool unbuffered,
 
   // Get the starting position.
   off_t loc = ::lseek(FD, 0, SEEK_CUR);
-#ifdef _WIN32
-  // MSVCRT's _lseek(SEEK_CUR) doesn't return -1 for pipes.
   sys::fs::file_status Status;
   std::error_code EC = status(FD, Status);
-  SupportsSeeking = !EC && Status.type() == sys::fs::file_type::regular_file;
+  IsRegularFile = Status.type() == sys::fs::file_type::regular_file;
+#ifdef _WIN32
+  // MSVCRT's _lseek(SEEK_CUR) doesn't return -1 for pipes.
+  SupportsSeeking = !EC && IsRegularFile;
 #else
-  SupportsSeeking = loc != (off_t)-1;
+  SupportsSeeking = !EC && loc != (off_t)-1;
 #endif
   if (!SupportsSeeking)
     pos = 0;
@@ -914,8 +915,7 @@ raw_fd_stream::raw_fd_stream(StringRef Filename, std::error_code &EC)
   if (EC)
     return;
 
-  // Do not support non-seekable files.
-  if (!supportsSeeking())
+  if (!isRegularFile())
     EC = std::make_error_code(std::errc::invalid_argument);
 }