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exec: Do not use absolute address hints for code_gen_buffer with -fpie

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The hard-coded addresses inside alloc_code_gen_buffer only make sense
if we're building an executable that will actually run at the address
we've put into the linker scripts.

When we're building with -fpie, the executable will run at some
random location chosen by the kernel.  We get better placement for
the code_gen_buffer if we allow the kernel to place the memory,
as it will tend to to place it near the executable, based on the
PROT_EXEC bit.

Since code_gen_prologue is always inside the executable, this effect
is easily seen at the end of most TB, with the exit_tb opcode, and
with any calls to helper functions.

Signed-off-by: Richard Henderson <rth@twiddle.net>
Signed-off-by: Blue Swirl <blauwirbel@gmail.com>
This commit is contained in:
Richard Henderson 2012-10-16 17:30:12 +10:00 committed by Blue Swirl
parent 3d85a72fd8
commit 405def1846
1 changed files with 6 additions and 1 deletions
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7
exec.c
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@ -578,7 +578,12 @@ static inline void *alloc_code_gen_buffer(void)
/* Constrain the position of the buffer based on the host cpu.
Note that these addresses are chosen in concert with the
addresses assigned in the relevant linker script file. */
# if defined(__x86_64__) && defined(MAP_32BIT)
# if defined(__PIE__) || defined(__PIC__)
/* Don't bother setting a preferred location if we're building
a position-independent executable. We're more likely to get
an address near the main executable if we let the kernel
choose the address. */
# elif defined(__x86_64__) && defined(MAP_32BIT)
/* Force the memory down into low memory with the executable.
Leave the choice of exact location with the kernel. */
flags |= MAP_32BIT;