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RE: basic asm and memory clobbers - Proposed solution
- From: Bernd Edlinger <bernd dot edlinger at hotmail dot de>
- To: David Wohlferd <dw at LimeGreenSocks dot com>, "gcc at gcc dot gnu dot org" <gcc at gcc dot gnu dot org>
- Cc: Joseph Myers <joseph at codesourcery dot com>, "Paul_Koning at Dell dot com" <Paul_Koning at Dell dot com>, "jakub at redhat dot com" <jakub at redhat dot com>, "rth at gcc dot gnu dot org" <rth at gcc dot gnu dot org>, "rth at redhat dot com" <rth at redhat dot com>, "pinskia at gcc dot gnu dot org" <pinskia at gcc dot gnu dot org>, Jeff Law <law at redhat dot com>, Segher Boessenkool <segher at kernel dot crashing dot org>, "aph at redhat dot com" <aph at redhat dot com>, Ian Lance Taylor <iant at google dot com>, Sandra Loosemore <sandra at codesourcery dot com>, Hans-Peter Nilsson <hp at bitrange dot com>
- Date: Sun, 29 Nov 2015 23:45:03 +0000
- Subject: RE: basic asm and memory clobbers - Proposed solution
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Hi,
> Well, I start to think that Jeff is right, and we should treat a asm ("") as if it
> were asm volatile ("" ::: )
> but if the asm ("nonempty with optional %") we should
> treat it as asm volatile ("nonempty with optional %%" ::: "memory").
> Our docs should say that explicitly, and the implementation should follow that
> direction.
>
attached is a patch that implements the above proposal.
I tried with this simple test case:
int x;
int main()
{
x = 1;
asm("test %eax,2");
x = 2;
}
gcc -O3 -S test.c
main:
.LFB0:
.cfi_startproc
movl $1, x(%rip)
#APP
# 5 "test.c" 1
test %eax,2
# 0 "" 2
#NO_APP
movl $2, x(%rip)
xorl %eax, %eax
ret
.cfi_endproc
What do you guys think?
Bernd.Index: gimple.c
===================================================================
--- gimple.c (Revision 230815)
+++ gimple.c (Arbeitskopie)
@@ -2567,6 +2567,10 @@
return true;
}
+ /* Non-empty basic ASM implicitly clobbers memory. */
+ if (gimple_asm_input_p (stmt) && strlen (gimple_asm_string (stmt)) != 0)
+ return true;
+
return false;
}
Index: cfgexpand.c
===================================================================
--- cfgexpand.c (Revision 230815)
+++ cfgexpand.c (Arbeitskopie)
@@ -2650,9 +2650,23 @@
{
rtx body;
- if (TREE_CODE (string) == ADDR_EXPR)
- string = TREE_OPERAND (string, 0);
+ /* Non-empty basic ASM implicitly clobbers memory. */
+ if (TREE_STRING_LENGTH (string) != 0)
+ {
+ rtx asm_op, clob;
+ asm_op = gen_rtx_ASM_OPERANDS (VOIDmode, empty_string, empty_string, 0,
+ rtvec_alloc (0), rtvec_alloc (0),
+ rtvec_alloc (0), locus);
+ MEM_VOLATILE_P (asm_op) = 1;
+
+ clob = gen_rtx_SCRATCH (VOIDmode);
+ clob = gen_rtx_MEM (BLKmode, clob);
+ clob = gen_rtx_CLOBBER (VOIDmode, clob);
+
+ emit_insn (gen_rtx_PARALLEL (VOIDmode, gen_rtvec (2, asm_op, clob)));
+ }
+
body = gen_rtx_ASM_INPUT_loc (VOIDmode,
ggc_strdup (TREE_STRING_POINTER (string)),
locus);