[RFC] Add middle end hook for stack red zone size

Jiangning Liu jiangning.liu@arm.com
Tue Jun 28 08:49:00 GMT 2011

This patch is to fix PR38644, which is a bug with long history about stack
red zone access, and PR30282 is correlated.

Originally red zone concept is not exposed to middle-end, and back-end uses
special logic to add extra memory barrier RTL and help the correct
dependence in middle-end. This way different back-ends must handle red zone
problem by themselves. For example, X86 target introduced function
ix86_using_red_zone() to judge red zone access, while POWER introduced
offset_below_red_zone_p() to judge it. Note that they have different
semantics, but the logic in caller sites of back-end uses them to decide
whether adding memory barrier RTL or not. If back-end incorrectly handles
this, bug would be introduced. 

Therefore, the correct method should be middle-end handles red zone related
things to avoid the burden in different back-ends. To be specific for
PR38644, this middle-end problem causes incorrect behavior for ARM target.
This patch exposes red zone concept to middle-end by introducing a
middle-end/back-end hook TARGET_STACK_RED_ZONE_SIZE defined in target.def,
and by default its value is 0. Back-end may redefine this function to
provide concrete red zone size according to specific ABI requirements. 

In middle end, scheduling dependence is modified by using this hook plus
checking stack frame pointer adjustment instruction to decide whether memory
references need to be all flushed out or not. In theory, if
TARGET_STACK_RED_ZONE_SIZE is defined correctly, back-end would not be
required to specially handle this scheduling dependence issue by introducing
extra memory barrier RTL.

In back-end, the following changes are made to define the hook,
1) For X86, TARGET_STACK_RED_ZONE_SIZE is redefined to be
ix86_stack_red_zone_size() in i386.c, which is an newly introduced function.
2) For POWER, TARGET_STACK_RED_ZONE_SIZE is redefined to be
rs6000_stack_red_zone_size() in rs6000.c, which is also a newly defined
3) For ARM and others, TARGET_STACK_RED_ZONE_SIZE is defined to be
default_stack_red_zone_size in targhooks.c, and this function returns 0,
which means ARM eabi and others don't support red zone access at all.

In summary, the relationship between ABI and red zone access is like below,

|   ARCH       |  ARM  |       X86     |    POWER      | others |
|    ABI       | EABI  | MS_64 | other |   AIX  |  V4  |        |
| RED ZONE     |  No   |  YES  |  No   |  YES   |  No  |   No   |
| RED ZONE SIZE|   0   |  128  |   0   |220/288 |   0  |    0   |

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