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aco: Don't exclude discardable parameters from register preservation

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The original semantic of discardable parameters was "okay, nothing
actually uses this parameter, feel free to clobber it", but we were
only using it with tail calls from a function without discardable
parameters, which was broken.

Instead, slightly change the use-case and utilize the "discardable"
attribute to mark parameters that the callee will clobber in a tail
call. This makes doing tail calls safe when the tail callee receives a
modified set of parameters.

Cc: mesa-stable
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/39579>
(cherry picked from commit ad23e02a28)
This commit is contained in:
Natalie Vock 2026-01-28 12:05:36 +01:00 • committed by Eric Engestrom
parent 7b1c9adfea
commit 0753012766
3 changed files with 10 additions and 6 deletions
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2
.pick_status.json
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@ -1634,7 +1634,7 @@
"description": "aco: Don't exclude discardable parameters from register preservation",
"nominated": true,
"nomination_type": 1,
"resolution": 0,
"resolution": 1,
"main_sha": null,
"because_sha": null,
"notes": null

8
src/amd/compiler/aco_nir_call_attribs.h
View file

@ -22,7 +22,13 @@ enum aco_nir_function_attribs {
};
enum aco_nir_parameter_attribs {
/* Parameter value is not used by any callee and does not need to be preserved */
/* This parameter's value may not be preserved across a callee. Unlike return parameters, the
* parameter's value is undefined on return. Callers must back up values of discardable
* parameters separately.
* Mostly used for tail calls, where parameters to the tail callee have different values than
* for the caller. In that case, on function return, the parameters will have been overwritten
* with the tail callee parameter values.
*/
ACO_NIR_PARAM_ATTRIB_DISCARDABLE = 0x1,
};

6
src/amd/compiler/instruction_selection/aco_isel_helpers.cpp
View file

@ -1034,8 +1034,7 @@ find_param_regs(Program* program, const ABI& abi, callee_info& info,
param_demand += Temp(0, it2->rc);
it2->dst_info->needs_explicit_preservation =
regs == clobbered_regs && !it2->dst_info->discardable;
it2->dst_info->needs_explicit_preservation = regs == clobbered_regs;
it2->dst_info->def.setPrecolored(*next_reg);
for (unsigned i = 0; i < it2->rc.size(); ++i)
BITSET_CLEAR(regs, next_reg->reg() + i);
@ -1051,8 +1050,7 @@ find_param_regs(Program* program, const ABI& abi, callee_info& info,
next_reg = next_reg->advance(required_padding * 4);
}
if (next_reg) {
params.back().dst_info->needs_explicit_preservation =
regs == clobbered_regs && !params.back().dst_info->discardable;
params.back().dst_info->needs_explicit_preservation = regs == clobbered_regs;
param_demand += Temp(0, params.back().rc);
params.back().dst_info->def.setPrecolored(*next_reg);
BITSET_CLEAR_COUNT(regs, next_reg->reg(), params.back().rc.size());