[codegen] Use VLQ for RelocInfo

Use the general VLQ implementation for RelocInfo, rather than a custom
varint implementation. Note that this reverses the tagging logic (from
end bit to continuation bit) as this is more efficient for small values
(that now don't need to be tagged/untagged when written/read).

Change-Id: I09283f0557f9e182b4466858a2786922affb1453
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2839555
Commit-Queue: Leszek Swirski <leszeks@chromium.org>
Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Cr-Commit-Position: refs/heads/master@{#74089}

src/codegen/reloc-info.cc

@@ -4,6 +4,7 @@
 
 #include "src/codegen/reloc-info.h"
 
+#include "src/base/vlq.h"
 #include "src/codegen/assembler-inl.h"
 #include "src/codegen/code-reference.h"
 #include "src/codegen/external-reference-encoder.h"
@@ -56,11 +57,10 @@ const char* const RelocInfo::kFillerCommentString = "DEOPTIMIZATION PADDING";
 //  the following record in the usual way. The long pc jump record has variable
 //  length:
 //               pc-jump:        [PC_JUMP] 11
-//                               [7 bits data] 0
+//                               1 [7 bits data]
 //                                  ...
-//                               [7 bits data] 1
-//               (Bits 6..31 of pc delta, with leading zeroes
-//                dropped, and last non-zero chunk tagged with 1.)
+//                               0 [7 bits data]
+//               (Bits 6..31 of pc delta, encoded with VLQ.)
 
 const int kTagBits = 2;
 const int kTagMask = (1 << kTagBits) - 1;
@@ -75,12 +75,6 @@ const int kSmallPCDeltaBits = kBitsPerByte - kTagBits;
 const int kSmallPCDeltaMask = (1 << kSmallPCDeltaBits) - 1;
 const int RelocInfo::kMaxSmallPCDelta = kSmallPCDeltaMask;
 
-const int kChunkBits = 7;
-const int kChunkMask = (1 << kChunkBits) - 1;
-const int kLastChunkTagBits = 1;
-const int kLastChunkTagMask = 1;
-const int kLastChunkTag = 1;
-
 uint32_t RelocInfoWriter::WriteLongPCJump(uint32_t pc_delta) {
   // Return if the pc_delta can fit in kSmallPCDeltaBits bits.
   // Otherwise write a variable length PC jump for the bits that do
@@ -89,13 +83,12 @@ uint32_t RelocInfoWriter::WriteLongPCJump(uint32_t pc_delta) {
   WriteMode(RelocInfo::PC_JUMP);
   uint32_t pc_jump = pc_delta >> kSmallPCDeltaBits;
   DCHECK_GT(pc_jump, 0);
-  // Write kChunkBits size chunks of the pc_jump.
-  for (; pc_jump > 0; pc_jump = pc_jump >> kChunkBits) {
-    byte b = pc_jump & kChunkMask;
-    *--pos_ = b << kLastChunkTagBits;
-  }
-  // Tag the last chunk so it can be identified.
-  *pos_ = *pos_ | kLastChunkTag;
+  base::VLQEncodeUnsigned(
+      [this](byte byte) {
+        *--pos_ = byte;
+        return pos_;
+      },
+      pc_jump);
   // Return the remaining kSmallPCDeltaBits of the pc_delta.
   return pc_delta & kSmallPCDeltaMask;
 }
@@ -205,14 +198,8 @@ void RelocIterator::AdvanceReadData() {
 
 void RelocIterator::AdvanceReadLongPCJump() {
   // Read the 32-kSmallPCDeltaBits most significant bits of the
-  // pc jump in kChunkBits bit chunks and shift them into place.
-  // Stop when the last chunk is encountered.
-  uint32_t pc_jump = 0;
-  for (int i = 0; i < kIntSize; i++) {
-    byte pc_jump_part = *--pos_;
-    pc_jump |= (pc_jump_part >> kLastChunkTagBits) << i * kChunkBits;
-    if ((pc_jump_part & kLastChunkTagMask) == 1) break;
-  }
+  // pc jump as a VLQ encoded integer.
+  uint32_t pc_jump = base::VLQDecodeUnsigned([this] { return *--pos_; });
   // The least significant kSmallPCDeltaBits bits will be added
   // later.
   rinfo_.pc_ += pc_jump << kSmallPCDeltaBits;