[PATCH] include: sbi: Optimize reads of mhartid and mscratch

[Samuel Holland]

csr_read() is marked as volatile and clobbering memory, which is
generally the safe thing to do. However, these two CSRs do not have any
side effects, and the values returned do not change between calls. The
compiler can generate better code if we allow it to reorder calls to
these functions and cache the return value. This change reduces code
size by about 200 bytes.

Signed-off-by: Samuel Holland <samuel.holland at sifive.com>
---

 include/sbi/riscv_asm.h   | 10 +++++++++-
 include/sbi/sbi_scratch.h | 11 +++++++++--
 2 files changed, 18 insertions(+), 3 deletions(-)

diff --git a/include/sbi/riscv_asm.h b/include/sbi/riscv_asm.h
index 2c34635a..a439f935 100644
--- a/include/sbi/riscv_asm.h
+++ b/include/sbi/riscv_asm.h
@@ -163,7 +163,15 @@ void csr_write_num(int csr_num, unsigned long val);
 	} while (0)
 
 /* Get current HART id */
-#define current_hartid()	((unsigned int)csr_read(CSR_MHARTID))
+static inline unsigned int current_hartid(void)
+{
+	unsigned int hartid;
+
+	/* Do not use csr_read() so the compiler can cache the value. */
+	__asm__ ("csrr %0, mhartid" : "=r"(hartid));
+
+	return hartid;
+}
 
 /* determine CPU extension, return non-zero support */
 int misa_extension_imp(char ext);
diff --git a/include/sbi/sbi_scratch.h b/include/sbi/sbi_scratch.h
index 12e6a983..28b6d7c9 100644
--- a/include/sbi/sbi_scratch.h
+++ b/include/sbi/sbi_scratch.h
@@ -154,8 +154,15 @@ enum sbi_scratch_options {
 };
 
 /** Get pointer to sbi_scratch for current HART */
-#define sbi_scratch_thishart_ptr() \
-	((struct sbi_scratch *)csr_read(CSR_MSCRATCH))
+static inline struct sbi_scratch *sbi_scratch_thishart_ptr(void)
+{
+	struct sbi_scratch *scratch;
+
+	/* Do not use csr_read() so the compiler can cache the value. */
+	__asm__ ("csrr %0, mscratch" : "=r"(scratch));
+
+	return scratch;
+}
 
 /** Get Arg1 of next booting stage for current HART */
 #define sbi_scratch_thishart_arg1_ptr() \
-- 
2.45.1