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[PULL,01/26] exec-all.h: Use stl_p to avoid undefined behaviour patching x86 jumps

Message ID 1399937474-6530-2-git-send-email-rth@twiddle.net
State Accepted
Commit 86360ad71df0070283469b8ae59f33cdd013501d
Headers show

Commit Message

Richard Henderson May 12, 2014, 11:30 p.m. UTC 
From: Peter Maydell <peter.maydell@linaro.org>

The code which patches x86 jump instructions assumes it can do an
unaligned write of a uint32_t. This is actually safe on x86, but it's
still undefined behaviour. We have infrastructure for doing efficient
unaligned accesses which doesn't engage in undefined behaviour, so
use it.

This is technically fractionally less efficient, at least with gcc 4.6;
instead of one instruction:
 7b2:   89 3e                   mov    %edi,(%rsi)
we get an extra spurious store to the stack slot:
 7b2:   89 7c 24 64             mov    %edi,0x64(%rsp)
 7b6:   89 3e                   mov    %edi,(%rsi)

Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Richard Henderson <rth@twiddle.net>
---
 include/exec/exec-all.h | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)
diff mbox

Patch

diff --git a/include/exec/exec-all.h b/include/exec/exec-all.h
index f9ac332..1c49a21 100644
--- a/include/exec/exec-all.h
+++ b/include/exec/exec-all.h
@@ -229,7 +229,7 @@  void ppc_tb_set_jmp_target(unsigned long jmp_addr, unsigned long addr);
 static inline void tb_set_jmp_target1(uintptr_t jmp_addr, uintptr_t addr)
 {
     /* patch the branch destination */
-    *(uint32_t *)jmp_addr = addr - (jmp_addr + 4);
+    stl_p((void*)jmp_addr, addr - (jmp_addr + 4));
     /* no need to flush icache explicitly */
 }
 #elif defined(__aarch64__)