[PATCH] Fixing line numbers of inlined allocas

[Wolfgang Pieb]

When a function is inlined (for example, as a result of it carrying the attribute alwaysinline), the cloned instructions receive the call site’s line number information (per r210459). After cloning, the cloned alloca instructions are moved to the caller’s entry block, retaining the call site’s line number information. 
When, subsequently, new instructions are generated at the start of the caller’s entry block (such as instructions generated by the stack-protector pass), they pick up the line number information from the entry block’s first instruction, which may be one of the cloned alloca instructions.

This can cause incorrect line number information being generated for the start of the calling function, with an inlined function’s call site being associated with the first stack-protector instruction, for example. Since the stack-protector instruction is likely deemed to be the end of the function prologue, breakpoints that are set using the function name will be set at the wrong location.

Example:

int __attribute__((always_inline)) foo()
{
   int arr[10];
   arr[0] = 5;
   int sum = 4;
   return sum;
}

extern void bar();

int main()
{
  bar();
  int i = foo();   <== breaking at ‘main’ will stop here because main’s function prologue gets foo’s call site’s line number.
  return i;
}



The proposed fix is to modify the cloned alloca instructions’ debug information when they are moved to the caller’s entry block. Instead of the call site’s information they would get the same information as the instructions already at the start  of the caller’s entry block, i.e. they place where they’re being moved to.

Note to the test case: the sspstrong attribute on the 'main' function is not necessary to make the test case pass, but if one were to generate a .o file the incorrect line numbers for the function prologue would not appear without the stack-protector instructions being generated.

http://reviews.llvm.org/D5401

Files:
  lib/Transforms/Utils/InlineFunction.cpp
  test/DebugInfo/debug-info-always-inline.ll
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