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| author | Lasse Collin <lasse.collin@tukaani.org> | 2024-02-17 16:01:32 +0200 |
|---|---|---|
| committer | Lasse Collin <lasse.collin@tukaani.org> | 2024-02-17 16:01:32 +0200 |
| commit | f1d6b88aefcced538403c5c2606ba57065b16e70 (patch) | |
| tree | d18d4669ddc851c7588e562d88ed28157d96a54c | |
| parent | liblzma: Wrap a line exceeding 80 chars. (diff) | |
| download | xz-f1d6b88aefcced538403c5c2606ba57065b16e70.tar.xz | |
liblzma: Avoid implementation-defined behavior in the RISC-V filter.
GCC docs promise that it works and a few other compilers do
too. Clang/LLVM is documented source code only but unsurprisingly
it behaves the same as others on x86-64 at least. But the
certainly-portable way is good enough here so use that.
| -rw-r--r-- | src/liblzma/simple/riscv.c | 30 |
1 files changed, 22 insertions, 8 deletions
diff --git a/src/liblzma/simple/riscv.c b/src/liblzma/simple/riscv.c index 7b30da83..aabbb052 100644 --- a/src/liblzma/simple/riscv.c +++ b/src/liblzma/simple/riscv.c @@ -511,15 +511,29 @@ riscv_encode(void *simple lzma_attribute((__unused__)), // be the same. // Arithmetic right shift makes sign extension - // trivial but C doesn't guarantee it for - // signed integers so a fallback is provided - // for portability. + // trivial but (1) it's implementation-defined + // behavior (C99/C11/C23 6.5.7-p5) and so is + // (2) casting unsigned to signed (6.3.1.3-p3). + // + // One can check for (1) with + // + // if ((-1 >> 1) == -1) ... + // + // but (2) has to be checked from the + // compiler docs. GCC promises that (1) + // and (2) behave in the common expected + // way and thus + // + // addr += (uint32_t)( + // (int32_t)inst2 >> 20); + // + // does the same as the code below. But since + // the 100 % portable way is only a few bytes + // bigger code and there is no real speed + // difference, let's just use that, especially + // since the decoder doesn't need this at all. uint32_t addr = inst & 0xFFFFF000; - if ((-1 >> 1) == -1) - addr += (uint32_t)( - (int32_t)inst2 >> 20); - else - addr += (inst2 >> 20) + addr += (inst2 >> 20) - ((inst2 >> 19) & 0x1000); addr += now_pos + (uint32_t)i; |