[llvm] [Support] Add llvm::support::endian::writeNext (PR #88685)

[Fangrui Song via llvm-commits]

https://github.com/MaskRay created https://github.com/llvm/llvm-project/pull/88685

`writeNext` overloads increase the pointer argument like `readNext`.
Code like the following
```
endian::write32<ELFT::Endianness>(p, 42);
p += 4;
endian::write32<ELFT::Endianness>(p, 43);
p += 4;
```

can be simplified to:
```
endian::writeNext<uint32_t, ELFT::Endianness>(p, 42);
endian::writeNext<uint32_t, ELFT::Endianness>(p, 43);
```


>From e4cc675973b042597a06cc90c6f997bc3d76af8a Mon Sep 17 00:00:00 2001
From: Fangrui Song <i at maskray.me>
Date: Sun, 14 Apr 2024 23:18:37 -0700
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Created using spr 1.3.5-bogner
---
 llvm/include/llvm/Support/Endian.h    | 15 +++++++++++++++
 llvm/unittests/Support/EndianTest.cpp | 23 +++++++++++++++++++++++
 2 files changed, 38 insertions(+)

diff --git a/llvm/include/llvm/Support/Endian.h b/llvm/include/llvm/Support/Endian.h
index 4c0405cf1e2f69..1cdb5ca0d5eaa1 100644
--- a/llvm/include/llvm/Support/Endian.h
+++ b/llvm/include/llvm/Support/Endian.h
@@ -102,6 +102,21 @@ inline void write(void *memory, value_type value) {
   write<value_type, alignment>(memory, value, endian);
 }
 
+/// Write a value of a particular endianness, and increment the buffer past that
+/// value.
+template <typename value_type, std::size_t alignment = unaligned,
+          typename CharT>
+inline void writeNext(CharT *&memory, value_type value, endianness endian) {
+  write(memory, value, endian);
+  memory += sizeof(value_type);
+}
+
+template <typename value_type, endianness endian,
+          std::size_t alignment = unaligned, typename CharT>
+inline void writeNext(CharT *&memory, value_type value) {
+  writeNext<value_type, alignment, CharT>(memory, value, endian);
+}
+
 template <typename value_type>
 using make_unsigned_t = std::make_unsigned_t<value_type>;
 
diff --git a/llvm/unittests/Support/EndianTest.cpp b/llvm/unittests/Support/EndianTest.cpp
index ab7dfc3800691d..bba1a56168f709 100644
--- a/llvm/unittests/Support/EndianTest.cpp
+++ b/llvm/unittests/Support/EndianTest.cpp
@@ -36,6 +36,29 @@ TEST(Endian, Read) {
                                                                   1)));
 }
 
+TEST(Endian, WriteNext) {
+  unsigned char bigval[] = {0x00, 0x00}, *p = bigval;
+  endian::writeNext<int16_t, llvm::endianness::big>(p, short(0xaabb));
+  EXPECT_EQ(bigval[0], 0xaa);
+  EXPECT_EQ(bigval[1], 0xbb);
+  EXPECT_EQ(p, bigval + 2);
+
+  char littleval[8] = {}, *q = littleval;
+  endian::writeNext<uint32_t, llvm::endianness::little>(q, 0x44556677);
+  EXPECT_EQ(littleval[0], 0x77);
+  EXPECT_EQ(littleval[1], 0x66);
+  EXPECT_EQ(littleval[2], 0x55);
+  EXPECT_EQ(littleval[3], 0x44);
+  EXPECT_EQ(q, littleval + 4);
+
+  endian::writeNext<uint32_t>(q, 0x11223344, llvm::endianness::little);
+  EXPECT_EQ(littleval[4], 0x44);
+  EXPECT_EQ(littleval[5], 0x33);
+  EXPECT_EQ(littleval[6], 0x22);
+  EXPECT_EQ(littleval[7], 0x11);
+  EXPECT_EQ(q, littleval + 8);
+}
+
 TEST(Endian, ReadBitAligned) {
   // Simple test to make sure we properly pull out the 0x0 word.
   unsigned char littleval[] = {0x3f, 0x00, 0x00, 0x00, 0xc0, 0xff, 0xff, 0xff};