[llvm-bugs] [Bug 25776] New: llvm missing opportunities to overlap non-interfering local variables

via llvm-bugs llvm-bugs at lists.llvm.org
Tue Dec 8 09:16:50 PST 2015


            Bug ID: 25776
           Summary: llvm missing opportunities to overlap non-interfering
                    local variables
           Product: new-bugs
           Version: trunk
          Hardware: PC
                OS: Linux
            Status: NEW
          Severity: normal
          Priority: P
         Component: new bugs
          Assignee: unassignedbugs at nondot.org
          Reporter: thanm at google.com
                CC: llvm-bugs at lists.llvm.org
    Classification: Unclassified

Created attachment 15416
  --> https://llvm.org/bugs/attachment.cgi?id=15416&action=edit

When I compile the attached testcase "stack-ifthen.cc" with "clang++ -g -O"
using LLVM trunk (linux x86-64), I observe that the size of the stack frame is
larger than it needs to be (~1024 bytes).

Compiling with "g++-4.8 -g -O" yields a stack frame size of ~512 bytes; gcc is
able to recognize that the two local arrays can share the same stack space.
Clang/llvm creates a separate stack allocation for each array, resulting in a
larger stack frame size.

Interestingly, gcc/g++ appears to overlap the two arrays even when compiling
with -O0, which leads me to assume the layout is being done in the front end as
opposed to in the back end.

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