[PATCH] D16427: FastISel should flush local value map

[Wolfgang Pieb via llvm-commits]

wolfgangp created this revision.
wolfgangp added a reviewer: echristo.
wolfgangp added a subscriber: llvm-commits.
Herald added subscribers: dsanders, qcolombet.

When FastISel hands over code selection to SelectionDAGISel, it should flush the local value map in order to avoid pushing local value instructions to the top of the basic block, which may cause unnecessary spills in the presence of calls.

Example:
```
struct X
{
   int F();
};

void foo();

int main()
{
    foo();
    int (X::*mp)() = &X::F; // FastISel quits because of this store
    return 0;
}
```
generates the following:
    subq $32, %rsp
    xorl %eax, %eax <== local value instruction, spill occurs
    movl $0, -4(%rbp)
    movl %eax, -28(%rbp) # 4-byte Spill
    callq _Z3foov
    movq $0, -16(%rbp)
    movq $_ZN1X1FEv, -24(%rbp)
    movl -28(%rbp), %eax # 4-byte Reload
    addq $32, %rsp
    popq %rbp
    retq

with the proposed fix, the code is
    subq    $32, %rsp
    movl    $0, -4(%rbp)
    callq   _Z3foov
    movq    $0, -16(%rbp)
    movq    $_ZN1X1FEv, -24(%rbp)
    xorl    %eax, %eax    <=== local value instruction now after the call
    addq    $32, %rsp
    popq    %rbp
    retq

Two test cases needed adjustment:
1) test/CodeGen/Mips/emergency-spill-slot-near-fp.ll
Returning %argc instead of 0 ensures that the test case needs to emergency-spill the %gpr scratch register. Otherwise the code gen improvement from the FastISel change would render it unnecessary, interfering with the goal of the test.
2) test/DebugInfo/X86/vla.ll
The original case narrowly checked for %RDX and DW_OP_breg1. Make this a bit more flexible, otherwise different register assignments can lead to failures.


http://reviews.llvm.org/D16427

Files:
  include/llvm/CodeGen/FastISel.h
  lib/CodeGen/SelectionDAG/SelectionDAGISel.cpp
  test/CodeGen/Mips/emergency-spill-slot-near-fp.ll
  test/CodeGen/X86/fast-isel-flush.ll
  test/DebugInfo/X86/vla.ll

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