[PATCH v3 5/7] RISC-V: remove decrement/increment dance in ISA string parser

Conor Dooley
2023-06-07T20:29:20Z

While expanding on the comments in the ISA string parsing code, I
noticed that the conditional decrement of `isa` at the end of the loop
was a bit odd.
The parsing code expects that at the start of the for loop, `isa` will
point to the first character of the next unparsed extension.
However, depending on what the next extension is, this may not be true.
Unless the next extension is a multi-letter extension preceded by an
underscore, `isa` will either point to the string's null-terminator or
to the first character of the next extension, once the switch statement
has been evaluated.
Obviously incrementing `isa` at the end of the loop could cause it to
increment past the null terminator or miss a single letter extension, so
`isa` is conditionally decremented, just so that the loop can increment
it again.

It's easier to understand the code if, instead of this decrement +
increment dance, we instead use a while loop & rely on the handling of
individual extension types to leave `isa` pointing to the first
character of the next extension.
As already mentioned, this won't be the case where the following
extension is multi-letter & preceded by an underscore. To handle that,
invert the check and increment rather than decrement.
Hopefully this eliminates a "huh?!?" moment the next time somebody tries
to understand this code.

Reviewed-by: Andrew Jones <ajones@ventanamicro.com>
Signed-off-by: Conor Dooley <conor.dooley@microchip.com>
---
arch/riscv/kernel/cpufeature.c | 14 ++++++--------
1 file changed, 6 insertions(+), 8 deletions(-)

diff --git a/arch/riscv/kernel/cpufeature.c b/arch/riscv/kernel/cpufeature.c
index 7dd4589e79a4..84dc44a3e6e5 100644
--- a/arch/riscv/kernel/cpufeature.c
+++ b/arch/riscv/kernel/cpufeature.c
@@ -157,7 +157,7 @@ void __init riscv_fill_hwcap(void)
isa += 4;

bitmap_zero(this_isa, RISCV_ISA_EXT_MAX);
- for (; *isa; ++isa) {
+ while (*isa) {
const char *ext = isa++;
const char *ext_end = isa;
bool ext_long = false, ext_err = false;
@@ -270,14 +270,12 @@ void __init riscv_fill_hwcap(void)

/*
* The parser expects that at the start of an iteration isa points to the
- * character before the start of the next extension. This will not be the
- * case if we have just parsed a single-letter extension and the next
- * extension is not a multi-letter extension prefixed with an "_". It is
- * also not the case at the end of the string, where it will point to the
- * terminating null character.
+ * first character of the next extension. As we stop parsing an extension
+ * on meeting a non-alphanumeric character, an extra increment is needed
+ * where the succeeding extension is a multi-letter prefixed with an "_".
*/
- if (*isa != '_')
- --isa;
+ if (*isa == '_')
+ ++isa;

#define SET_ISA_EXT_MAP(name, bit) \
do { \
--
2.39.2