[llvm-bugs] [Bug 26333] New: [x86] Use pop to restore stack instead of add

[via llvm-bugs]

https://llvm.org/bugs/show_bug.cgi?id=26333

            Bug ID: 26333
           Summary: [x86] Use pop to restore stack instead of add
           Product: libraries
           Version: trunk
          Hardware: PC
                OS: Linux
            Status: NEW
          Severity: normal
          Priority: P
         Component: Backend: X86
          Assignee: unassignedbugs at nondot.org
          Reporter: hans at chromium.org
                CC: llvm-bugs at lists.llvm.org
            Blocks: 26299
    Classification: Unclassified

Consider the following at -Os:

  struct S {
    void foo();
  };

  S *bar(int);

  void baz() {
    bar(0)->foo();
  }

Clang will generate:

00000000 <?baz@@YAXXZ>:
   0:    6a 00                    push   $0x0
   2:    e8 00 00 00 00           call   7 <?baz@@YAXXZ+0x7>
   7:    83 c4 04                 add    $0x4,%esp
   a:    89 c1                    mov    %eax,%ecx
   c:    e9 00 00 00 00           jmp    11 <@feat.00+0x10>

Instead of 'add $0x4,%esp', MSVC would do 'pop %ecx' and save two bytes,
because %ecx isn't live there anyway.

This incurs a memory access, but since nothing depends on the result, maybe
that would be OK, at least at -Os?

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