https://gcc.gnu.org/bugzilla/show_bug.cgi?id=103149
--- Comment #3 from CVS Commits <cvs-commit at gcc dot gnu.org> --- The master branch has been updated by Alexandre Oliva <aol...@gcc.gnu.org>: https://gcc.gnu.org/g:0485ce9128f272fe7ccb48672f69d7be6bb26a9c commit r12-5831-g0485ce9128f272fe7ccb48672f69d7be6bb26a9c Author: Alexandre Oliva <ol...@adacore.com> Date: Sat Dec 4 00:17:18 2021 -0300 [PR103149] detach values through mem only if general regs won't do When hardening compares or conditional branches, we perform redundant tests, and to prevent them from being optimized out, we use asm statements that preserve a value used in a compare, but in a way that the compiler can no longer assume it's the same value, so it can't optimize the redundant test away. We used to use +g, but that requires general regs or mem. You might think that, if a reg constraint can't be satisfied, the register allocator will fall back to memory, but that's not so: we decide on matching MEMs very early on, by using the same addressable operand on both input and output, and only if the constraint does not allow registers. If it does, we use gimple registers and then pseudos as inputs and outputs, and then inputs can be substituted by equivalent expressions, and then, if no register contraint fits (e.g. because that mode won't fit in general regs, or won't fit in regs at all), the register allocator will give up before even trying to allocate some temporary memory to unify input and output. This patch arranges for us to create and use the temporary stack slot if we can tell the mode requires memory, or won't otherwise fit in general regs, and thus to use +m for that asm. for gcc/ChangeLog PR middle-end/103149 * gimple-harden-conditionals.cc (detach_value): Use memory if general regs won't do. for gcc/testsuite/ChangeLog PR middle-end/103149 * gcc.target/aarch64/pr103149.c: New.
