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intel/brw: Don't apply discard_if condition opt if it can change results

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We can't just always negate the alu instruction's cmod, because negating
it can produce different results when the argument is NaN float. We can
still do that if the condition is == or !=.

Fixes: 0ba9497e ("intel/fs: Improve discard_if code generation")
Closes: https://gitlab.freedesktop.org/mesa/mesa/-/issues/11800
Signed-off-by: Sviatoslav Peleshko <sviatoslav.peleshko@globallogic.com>
Reviewed-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/31042>
This commit is contained in:
Sviatoslav Peleshko 2024-09-05 13:41:19 +03:00 • committed by Marge Bot
parent 93372ea9af
commit 57344052b6
1 changed files with 21 additions and 1 deletions
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22
src/intel/compiler/brw_fs_nir.cpp
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@ -4235,8 +4235,28 @@ fs_nir_emit_fs_intrinsic(nir_to_brw_state &ntb,
/* The old sequence that would have been generated is,
* basically, bool_result == false. This is equivalent to
* !bool_result, so negate the old modifier.
*
* Unfortunately, we can't do this to most float comparisons
* because of NaN, so we'll have to fallback to the old-style
* compare.
*
* For example, this code (after negation):
* (+f1.0) cmp.ge.f1.0(8) null<1>F g30<8,8,1>F 0x0F
* will provide different results from this:
* cmp.l.f0.0(8) g31<1>F g30<1,1,0>F 0x0F
* (+f1.0) cmp.z.f1.0(8) null<1>D g31<8,8,1>D 0D
* because both (NaN >= 0) == false and (NaN < 0) == false.
*
* It will still work for == and != though, because
* (NaN == x) == false and (NaN != x) == true.
*/
cmp->conditional_mod = brw_negate_cmod(cmp->conditional_mod);
if (brw_type_is_float(cmp->src[0].type) &&
cmp->conditional_mod != BRW_CONDITIONAL_EQ &&
cmp->conditional_mod != BRW_CONDITIONAL_NEQ) {
cmp = NULL;
} else {
cmp->conditional_mod = brw_negate_cmod(cmp->conditional_mod);
}
}
}