[PATCH] avoid strict-aliasing warnings in gdbserver/linux-x86-low.c

[PATCH] avoid strict-aliasing warnings in gdbserver/linux-x86-low.c

[Nathan Froyd]

I was compiling with a combined tree this evening when the build stopped
with:

/home/froydnj/src/combined-tree/gdb/gdbserver/linux-x86-low.c: In function ‘amd64_emit_const’:
/home/froydnj/src/combined-tree/gdb/gdbserver/linux-x86-low.c:1822:3:
error: dereferencing type-punned pointer will break strict-aliasing rules [-Werror=strict-aliasing]

...and several other similar errors.  I don't know why, exactly; I've
been compiling with a combined tree for quite some time and this is the
first problem I've had.

Anyway, the below patch addresses the problem by using memcpy instead,
which is transformed into the same code by the compiler.

Tested on x86_64-unknown-linux-gnu.  OK to commit?

-Nathan

gdb/gdbserver/
	* linux-x86-low.c (amd64_emit_const): Call memcpy instead of
	casting pointers.
	(amd64_emit_reg, amd64_emit_int_call_1, amd64_emit_void_call_2):
	(i386_emit_const, i386_emit_reg, i386_emit_int_call_1):
	(i386_emit_void_call_2): Likewise.

diff --git a/gdb/gdbserver/linux-x86-low.c b/gdb/gdbserver/linux-x86-low.c
index 88f177d..027a563 100644
--- a/gdb/gdbserver/linux-x86-low.c
+++ b/gdb/gdbserver/linux-x86-low.c
@@ -1819,7 +1819,7 @@ amd64_emit_const (LONGEST num)
 
   i = 0;
   buf[i++] = 0x48;  buf[i++] = 0xb8; /* mov $<n>,%rax */
-  *((LONGEST *) (&buf[i])) = num;
+  memcpy (&buf[i], &num, sizeof (num));
   i += 8;
   append_insns (&buildaddr, i, buf);
   current_insn_ptr = buildaddr;
@@ -1876,7 +1876,7 @@ amd64_emit_reg (int reg)
   buildaddr = current_insn_ptr;
   i = 0;
   buf[i++] = 0xbe; /* mov $<n>,%esi */
-  *((int *) (&buf[i])) = reg;
+  memcpy (&buf[i], &reg, sizeof (reg));
   i += 4;
   append_insns (&buildaddr, i, buf);
   current_insn_ptr = buildaddr;
@@ -1959,7 +1959,7 @@ amd64_emit_int_call_1 (CORE_ADDR fn, int arg1)
   buildaddr = current_insn_ptr;
   i = 0;
   buf[i++] = 0xbf; /* movl $<n>,%edi */
-  *((int *) (&buf[i])) = arg1;
+  memcpy (&buf[i], &arg1, sizeof (arg1));
   i += 4;
   append_insns (&buildaddr, i, buf);
   current_insn_ptr = buildaddr;
@@ -1978,7 +1978,7 @@ amd64_emit_void_call_2 (CORE_ADDR fn, int arg1)
   buildaddr = current_insn_ptr;
   i = 0;
   buf[i++] = 0xbf; /* movl $<n>,%edi */
-  *((int *) (&buf[i])) = arg1;
+  memcpy (&buf[i], &arg1, sizeof (arg1));
   i += 4;
   append_insns (&buildaddr, i, buf);
   current_insn_ptr = buildaddr;
@@ -2301,18 +2301,19 @@ static void
 i386_emit_const (LONGEST num)
 {
   unsigned char buf[16];
-  int i, hi;
+  int i, hi, lo;
   CORE_ADDR buildaddr = current_insn_ptr;
 
   i = 0;
   buf[i++] = 0xb8; /* mov $<n>,%eax */
-  *((int *) (&buf[i])) = (num & 0xffffffff);
+  lo = num & 0xffffffff;
+  memcpy (&buf[i], &lo, sizeof (lo));
   i += 4;
   hi = ((num >> 32) & 0xffffffff);
   if (hi)
     {
       buf[i++] = 0xbb; /* mov $<n>,%ebx */
-      *((int *) (&buf[i])) = hi;
+      memcpy (&buf[i], &hi, sizeof (hi));
       i += 4;
     }
   else
@@ -2351,7 +2352,7 @@ i386_emit_reg (int reg)
   buildaddr = current_insn_ptr;
   i = 0;
   buf[i++] = 0xb8; /* mov $<n>,%eax */
-  *((int *) (&buf[i])) = reg;
+  memcpy (&buf[i], &reg, sizeof (reg));
   i += 4;
   append_insns (&buildaddr, i, buf);
   current_insn_ptr = buildaddr;
@@ -2451,7 +2452,7 @@ i386_emit_int_call_1 (CORE_ADDR fn, int arg1)
   buf[i++] = 0xc7;  /* movl $<arg1>,(%esp) */
   buf[i++] = 0x04;
   buf[i++] = 0x24;
-  *((int *) (&buf[i])) = arg1;
+  memcpy (&buf[i], &arg1, sizeof (arg1));
   i += 4;
   append_insns (&buildaddr, i, buf);
   current_insn_ptr = buildaddr;
@@ -2486,7 +2487,7 @@ i386_emit_void_call_2 (CORE_ADDR fn, int arg1)
   buf[i++] = 0xc7;  /* movl $<arg1>,(%esp) */
   buf[i++] = 0x04;
   buf[i++] = 0x24;
-  *((int *) (&buf[i])) = arg1;
+  memcpy (&buf[i], &arg1, sizeof (arg1));
   i += 4;
   append_insns (&buildaddr, i, buf);
   current_insn_ptr = buildaddr;

Re: [PATCH] avoid strict-aliasing warnings in gdbserver/linux-x86-low.c

[Pedro Alves]

On Wednesday 27 April 2011 02:40:14, Nathan Froyd wrote:
> Tested on x86_64-unknown-linux-gnu.  OK to commit?

Okay, thanks!

> gdb/gdbserver/
>         * linux-x86-low.c (amd64_emit_const): Call memcpy instead of
>         casting pointers.
>         (amd64_emit_reg, amd64_emit_int_call_1, amd64_emit_void_call_2):
>         (i386_emit_const, i386_emit_reg, i386_emit_int_call_1):
>         (i386_emit_void_call_2): Likewise.

-- 
Pedro Alves