nir: Add ACCESS_CAN_SPECULATE

Determining whether it is safe to hoist a load instruction out of control flow
depends on complex hardware and driver details. Rather than encoding this as
knobs in every NIR pass that wants to do so (notably nir_opt_preamble and
nir_opt_peephole_select), add a per-load ACCESS flag for backends to set.

Signed-off-by: Alyssa Rosenzweig <alyssa@rosenzweig.io>
Reviewed-by: Connor Abbott <cwabbott0@gmail.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/24011>

diff --git a/src/compiler/nir/nir_print.c b/src/compiler/nir/nir_print.c
index 56d280c..6c7bc82 100644 (file)
--- a/src/compiler/nir/nir_print.c
+++ b/src/compiler/nir/nir_print.c
@@ -802,6 +802,7 @@ print_access(enum gl_access_qualifier access, print_state *state, const char *se
       { ACCESS_NON_WRITEABLE, "readonly" },
       { ACCESS_NON_READABLE, "writeonly" },
       { ACCESS_CAN_REORDER, "reorderable" },
+      { ACCESS_CAN_SPECULATE, "speculatable" },
       { ACCESS_NON_TEMPORAL, "non-temporal" },
       { ACCESS_INCLUDE_HELPERS, "include-helpers" },
    };

diff --git a/src/compiler/shader_enums.h b/src/compiler/shader_enums.h
index 31e8ebb..a49f4be 100644 (file)
--- a/src/compiler/shader_enums.h
+++ b/src/compiler/shader_enums.h
@@ -1111,6 +1111,17 @@ enum gl_access_qualifier
     * from fragment mask buffer.
     */
    ACCESS_FMASK_LOWERED_AMD = (1 << 11),
+
+   /**
+    * Whether it is safe to speculatively execute this load. This allows
+    * hoisting loads out of conditional control flow (including out of software
+    * bounds checks). Setting this optimally depends on knowledge of the
+    * hardware. Speculation is safe if out-of-bounds access does not trigger
+    * undefined behaviour (even though the returned value of the speculated load
+    * is bogus). This is the case if there is hardware-level bounds checking, or
+    * if MMU faults are suppressed for the load.
+    */
+   ACCESS_CAN_SPECULATE = (1 << 12),
 };
 
 /**