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Re: [PATCH] Account for prologue spills in reg_pressure scheduling
- From: Sebastian Pop <sebpop at gmail dot com>
- To: Maxim Kuvyrkov <maxim dot kuvyrkov at linaro dot org>
- Cc: GCC Patches <gcc-patches at gcc dot gnu dot org>, Vladimir Makarov <vmakarov at redhat dot com>, Richard Sandiford <rdsandiford at googlemail dot com>
- Date: Mon, 20 Oct 2014 19:11:05 +0000
- Subject: Re: [PATCH] Account for prologue spills in reg_pressure scheduling
- Authentication-results: sourceware.org; auth=none
- References: <FE7CEF4D-A27B-46D0-96B3-3569C1558DF9 at linaro dot org>
Maxim Kuvyrkov wrote:
> This patch improves register pressure scheduling (both SCHED_PRESSURE_WEIGHTED and SCHED_PRESSURE_MODEL) to better estimate number of available registers.
> At the moment the scheduler does not account for spills in the prologues and restores in the epilogue, which occur from use of call-used registers. The current state is, essentially, optimized for case when there is a hot loop inside the function, and the loop executes significantly more often than the prologue/epilogue. However, on the opposite end, we have a case when the function is just a single non-cyclic basic block, which executes just as often as prologue / epilogue, so spills in the prologue hurt performance as much as spills in the basic block itself. In such a case the scheduler should throttle-down on the number of available registers and try to not go beyond call-clobbered registers.
> The patch uses basic block frequencies to balance the cost of using call-used registers for intermediate cases between the two above extremes.
> The motivation for this patch was a floating-point testcase on arm-linux-gnueabihf (ARM is one of the few targets that use register pressure scheduling by default).
Does aarch64 enable reg pressure sched by default, or what is the flag to enable it?
I'm planing to look at the perf impact of the patch.