[llvm-bugs] [Bug 26876] New: llc generates wrong code for x86 with -march=atom

[via llvm-bugs]

https://llvm.org/bugs/show_bug.cgi?id=26876

            Bug ID: 26876
           Summary: llc generates wrong code for x86 with -march=atom
           Product: new-bugs
           Version: 3.7
          Hardware: PC
                OS: Linux
            Status: NEW
          Severity: normal
          Priority: P
         Component: new bugs
          Assignee: unassignedbugs at nondot.org
          Reporter: arne.hasselbring at googlemail.com
                CC: llvm-bugs at lists.llvm.org
    Classification: Unclassified

Created attachment 16004
  --> https://llvm.org/bugs/attachment.cgi?id=16004&action=edit
A minimal source code example that compiles to bad code

Compiling the attached file with the following command line gives a program
that crashes when returning from foo because of improper stack operations.

Compilation command line:
$ clang++ -std=c++14 -m32 -march=atom -o prog main.cpp

Executing prog results in
$ ./prog
Before foo()
Segmentation fault

This seems to happen if std::chrono::system_clock::now() is the last operation
in a function. It doesn't seem to matter whether the result is written into a
variable or not. Initially the problem occured when initializing the last
member variable of a class with now(). For this minimal example, I had to add
an __attribute__((noinline)) to foo() to get the error, but it is exactly the
same as in the original code.

I traced this down to an error in the generated assembly code. foo is compiled
to the following code:

 8048780:       55                      push   %ebp
 8048781:       89 e5                   mov    %esp,%ebp
 8048783:       8d 64 24 e8             lea    -0x18(%esp),%esp
 8048787:       8d 45 f8                lea    -0x8(%ebp),%eax
 804878a:       89 04 24                mov    %eax,(%esp)
 804878d:       e8 3e fe ff ff          call   80485d0
<_ZNSt6chrono3_V212system_clock3nowEv at plt>
 8048792:       8d 64 24 19             lea    0x19(%esp),%esp
 8048796:       5d                      pop    %ebp
 8048797:       c3                      ret    

You can see that in the beginning, 0x18 is subtracted from %esp and in the end,
0x19 is added. std::chrono::system_clock::now() adds another 4 bytes to %esp
(by returning with ret 0x04, I suspect this has to do with some ABI
callee-cleanup things), so in the end, the stack pointer is 5 bytes higher than
it should be. So it leads to a corrupted stack and the ret at 0x8048797 jumps
into some undefined place.
Compiling without -march=atom leads to a correct addition of 0x14 to %esp.

The clang/llvm is compiled by crosstool-ng, clang++ --version gives:
clang version 3.7.0 (https://github.com/llvm-mirror/clang.git
40b68b4c02b9d9e1e4138815747adf5589496240)
(https://github.com/llvm-mirror/llvm.git
17611b180b77406671cf0819a452dd128ce7481c)

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