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liblzma: Avoid implementation-defined behavior in the RISC-V filter.

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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.
This commit is contained in:
Lasse Collin 2024-02-17 16:01:32 +02:00
parent 843ddc5f61
commit f1d6b88aef
1 changed files with 22 additions and 8 deletions
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28
src/liblzma/simple/riscv.c
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@ -511,14 +511,28 @@ 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)
- ((inst2 >> 19) & 0x1000);