68d075e18b
The 3 bits that represent rsp are used as a signaling mechanism to choose between compact base + disp encoding and full base + disp + scale*index encoding, one byte longer. If a base is encoded as rsp in the MOD R/M byte, an SIB byte follows. r12 shares its 3 bottom bits with rsp, so we need to treat it like rsp when deciding whether or not we need an SIB byte. (As usual registers r8-15's distinguishing upper bit is carried by a REX/VEX prefix.) rsp is also treated as a special signal in the index field of the SIB byte, meaning essentially scale=0, and as a result it's not possible to use rsp as an index. It _is_ possible to use r12 as an index, with a test added here. This worked without any code change. It seems the index=rsp -> scale=0 signal is triggered by all four bits of the index registger, including the X upper bit from REX/VEX. I have found these charts useful: https://wiki.osdev.org/X86-64_Instruction_Encoding#32.2F64-bit_addressing_2 Looking at those charts I noticed that rbp/r13 are also special cases, so I'm eyeing them warily and will avoid using them for now. Change-Id: Id78f826a39c060b03000eae7c50c642ef44d57db Reviewed-on: https://skia-review.googlesource.com/c/skia/+/306237 Reviewed-by: Herb Derby <herb@google.com> Commit-Queue: Mike Klein <mtklein@google.com> |
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| animations | ||
| bench | ||
| bin | ||
| build/fuchsia | ||
| build_overrides | ||
| client_utils/android | ||
| demos.skia.org | ||
| dm | ||
| docker | ||
| docs/examples | ||
| example | ||
| experimental | ||
| fuzz | ||
| gm | ||
| gn | ||
| include | ||
| infra | ||
| modules | ||
| platform_tools | ||
| resources | ||
| samplecode | ||
| site | ||
| specs | ||
| src | ||
| tests | ||
| third_party | ||
| tools | ||
| .clang-format | ||
| .clang-tidy | ||
| .gitignore | ||
| .gn | ||
| AUTHORS | ||
| BUILD.gn | ||
| codereview.settings | ||
| CONTRIBUTING | ||
| CQ_COMMITTERS | ||
| DEPS | ||
| go.mod | ||
| go.sum | ||
| LICENSE | ||
| OWNERS | ||
| PRESUBMIT.py | ||
| public.bzl | ||
| README | ||
| README.chromium | ||
| RELEASE_NOTES.txt | ||
| whitespace.txt | ||
Skia is a complete 2D graphic library for drawing Text, Geometries, and Images. See full details, and build instructions, at https://skia.org.