tree 415b360f9598273275a2e7478340cd1c8cdf2d0a
parent 576161cb6069e2c7656a8ef530727a0f4aefff30
author Benjamin Maxwell <benjamin.maxwell@arm.com> 1745584389 +0100
committer GitHub <noreply@github.com> 1745584389 +0100
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[AArch64][SME] Allow spills of ZT0 around SME ABI routines again (#136726)

In #132722 spills of ZT0 were disabled around all SME ABI routines to
avoid a case where ZT0 is spilled before ZA is enabled (resulting in a
crash).

It turns out that the ABI does not promise that routines will preserve
ZT0 (however in practice they do), so generally disabling ZT0 spills for
ABI routines is not correct.

The case where a crash was possible was "aarch64_new_zt0" functions with
ZA disabled on entry and a ZT0 spill around __arm_tpidr2_save. In this
case, ZT0 will be undefined at the call to __arm_tpidr2_save, so this
patch avoids the ZT0 spill by marking the callsite with
"aarch64_zt0_undef". This attribute only applies to callsites and marks
that at the point the call is made ZT0 is not defined, so does not need
preserving.