[PATCH] c++ bugfix: inconsistent runtime beahavior when have -O2

["Hongbo Liu" <lhbf@qq.com>]

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Hi,

This is my first time using email patch, please correct me if there is any error. Thanks!


I found a c++ optimization bug first introduced via commit 520d5ad337eaa15860a5a964daf7ca46cf31c029, which will make program compiled with -O2 have unexpected behavior.


How to produce


1. Checkout to latest master commit: c4d702fb3c1e2f6e1bc8711da81bff59543b1b19
2. Build and install the gcc;
3. Compile the attached file `test.cc` with command: `latest-g++ -std=c++20 -g0 -O1 test.cc -o test_o1`;
4. The output of `./test_o1` will be `offset delta: 5`;
5.  Compile the attached file `test.cc` with command: `latest-g++ -std=c++20 -g0 -O2 test.cc -o test_o2`;
6. The output of `./test_o2` will be `offset delta: 0`;


The program behavior with `-O2` is inconsistent with `-O1` and unexpected.


How to fix


The bug was introduced via commit 520d5ad337eaa15860a5a964daf7ca46cf31c029. The attached patch file could fix the problem and make `-O2` and `-O1` have same behavior.


Request for suggestion


I could not find and document about how to add this kind of test, could anyone give me some suggestions?

[-- Attachment #2: test.cc --]
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#include <cstdint>
#include <iostream>
#include <algorithm>
#include <vector>

using namespace std;

void parse_varint_u64(void **buf, uint64_t *val) {
	size_t of = 0;
	uint64_t num = 0;

	do {
		num |= (uint64_t)(((char *)(*buf))[(int)of] & 0x7f);
	} while (((char *)(*buf))[(int)of++] & 0x80);

	*val = num;
}

void parse_varint_i64(void **buf, int64_t *val) {
	uint64_t n = 123;
	parse_varint_u64(buf, &n);

  // printf("n: %d\n", n);

  *val = (int64_t)(n >> 1) ^ -(int64_t)(n & 1);
}

int main(int argc, char *argv[]) {
  size_t size = 16;
  void* data = malloc(size);
  for (int i = 0; i < size; ++i) {
    ((char*)data)[i] = i + 10;
  }

	long long offset_delta = 0;

  parse_varint_i64(&data, (int64_t *)&offset_delta);

  std::cout << "offset delta: " << offset_delta << std::endl;

  return 0;
}

[-- Attachment #3: 0001-PATCH-c-bugfix-inconsistent-logic-when-have-O2.patch --]
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From 2cfe523be931f26aa9988fa4d0b8ca57be548517 Mon Sep 17 00:00:00 2001
From: HongboLiu <lhbf@qq.com>
Date: Mon, 6 Jun 2022 16:49:32 +0800
Subject: [PATCH] [PATCH] c++ bugfix: inconsistent logic when have -O2

---
 gcc/gimple.cc | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/gcc/gimple.cc b/gcc/gimple.cc
index b70ab4d..d9593aa 100644
--- a/gcc/gimple.cc
+++ b/gcc/gimple.cc
@@ -1566,6 +1566,9 @@ gimple_call_arg_flags (const gcall *stmt, unsigned arg)
   attr_fnspec fnspec = gimple_call_fnspec (stmt);
   int flags = 0;
 
+  if (!fnspec.known_p ())
+    return 0;
+
   if (fnspec.known_p ())
     flags = fnspec.arg_eaf_flags (arg);
   tree callee = gimple_call_fndecl (stmt);
-- 
2.7.4