IRA: Fix implicit preference calculation for hard registers.
This patch corrects an issue in the IRA that could lead to suboptimal code generation by improving the hard register preference calculation.
The IRA (Integrated Register Allocator) could ignore some valid register alternatives for subsequent operands when searching for preferred hard registers under strict constraints. This could result in suboptimal code generation because the allocator didn’t properly consider all options. This patch fixes the issue by ensuring that the same starting preference is used for all operands, leading to better register allocation decisions.
In Details
This commit modifies ira-lives.cc, specifically the ira_implicitly_set_insn_hard_regs function within the IRA. The issue stemmed from how the allocator considered preferred hard registers when faced with overly restrictive constraints. By ensuring a consistent starting point for preference consideration across all operands, the patch avoids prematurely discarding potentially beneficial register assignments. This optimization directly impacts register allocation quality.
For Context
Register allocation is a crucial process in compilation, where the compiler assigns program variables to physical registers in the CPU. The Integrated Register Allocator (IRA) is a component of GCC responsible for making these assignments. This patch addresses a problem in the IRA's decision-making process: under certain conditions, it could overlook some register choices, leading to less efficient code. The fix ensures a more thorough search for the best register assignments, improving overall code quality.