Hi!
As mentioned in the "inline asm: Add new constraint for symbol definitions"
patch description, while the c operand modifier is documented to:
Require a constant operand and print the constant expression with no
punctuation.
it actually doesn't do that with -fpic at least on some targets and
has been behaving that way for at least 3 decades.
It prints the operand using output_addr_const if CONSTANT_ADDRESS_P is true,
but CONSTANT_ADDRESS_P can do all sorts of target specific checks.
And if it is false, it falls back to output_operand (operands[opnum], 'c');
which will on various targets just result in an error that it is invalid
modifier letter (weird because it is documented), on others like x86 or
alpha will handle the operand in some weird way if it is a comparison
and otherwise complain the argument isn't a comparison, on others like
arm perhaps do what the user wanted.
As I wrote, we are pretty much out of modifier letters because some targets
use a lot of them, and almost out of % punctuation chars (I think ` is left)
but right now punctuation chars aren't normally followed by operand number
anyway.
So, the following patch takes one of the generic letters (c) and adds an
extra modifier char after it, I chose cc, which behaves like c but just
always uses output_addr_const instead of falling back to the machine
dependent code.
2024-11-06 Jakub Jelinek <ja...@redhat.com>
* final.cc (output_asm_insn): Add support for cc operand modifier.
* doc/extend.texi (Generic Operand Modifiers): Document cc operand
modifier.
* doc/md.texi (@samp{:} in constraint): Mention the cc operand
modifier and add small example.
* c-c++-common/toplevel-asm-4.c: Don't use -fno-pie option.
Use cc modifier instead of c.
(v, w): Add extern keyword.
* c-c++-common/toplevel-asm-6.c: New test.
--- gcc/final.cc.jj 2024-11-06 10:23:21.214728021 +0100
+++ gcc/final.cc 2024-11-06 11:02:40.406413277 +0100
@@ -3492,7 +3492,10 @@ output_asm_insn (const char *templ, rtx
int letter = *p++;
unsigned long opnum;
char *endptr;
+ int letter2 = 0;
+ if (letter == 'c' && *p == 'c')
+ letter2 = *p++;
opnum = strtoul (p, &endptr, 10);
if (endptr == p)
@@ -3506,7 +3509,7 @@ output_asm_insn (const char *templ, rtx
output_address (VOIDmode, operands[opnum]);
else if (letter == 'c')
{
- if (CONSTANT_ADDRESS_P (operands[opnum]))
+ if (letter2 == 'c' || CONSTANT_ADDRESS_P (operands[opnum]))
output_addr_const (asm_out_file, operands[opnum]);
else
output_operand (operands[opnum], 'c');
--- gcc/doc/extend.texi.jj 2024-11-06 10:23:21.220727937 +0100
+++ gcc/doc/extend.texi 2024-11-06 11:09:46.433407349 +0100
@@ -12113,6 +12113,11 @@ The following table shows the modifiers
@item @code{c}
@tab Require a constant operand and print the constant expression with no
punctuation.
@tab @code{%c0}
+@item @code{cc}
+@tab Like @samp{%c} except try harder to print it with no punctuation.
+@samp{%c} can e.g.@: fail to print constant addresses in position independent
code on
+some architectures.
+@tab @code{%cc0}
@item @code{n}
@tab Like @samp{%c} except that the value of the constant is negated before
printing.
@tab @code{%n0}
--- gcc/doc/md.texi.jj 2024-11-06 10:30:09.884948601 +0100
+++ gcc/doc/md.texi 2024-11-06 11:12:51.893792814 +0100
@@ -1510,6 +1510,13 @@ This constraint, allowed only in input o
pattern defines specific function or variable symbol. The constraint
shouldn't be mixed with other constraints on the same operand and
the operand should be address of a function or non-automatic variable.
+Best used with the @samp{cc} modifier when printing the operand, so that
+even in position independent code it prints as a label.
+
+@smallexample
+void foo (void);
+asm (".globl %cc0; %cc0: ret" : : ":" (foo));
+@end smallexample
@cindex other register constraints
@cindex extensible constraints
--- gcc/testsuite/c-c++-common/toplevel-asm-4.c.jj 2024-11-06
10:30:09.888948544 +0100
+++ gcc/testsuite/c-c++-common/toplevel-asm-4.c 2024-11-06 11:14:14.553627513
+0100
@@ -1,9 +1,8 @@
/* PR c/41045 */
/* { dg-do compile } */
/* { dg-options "-O0" } */
-/* { dg-additional-options "-fno-pie" { target pie } } */
-int v[42], w;
+extern int v[42], w;
void foo (void);
-asm ("# %c0: %c1:" :: ":" (foo), ":" (v), ":" (&w));
+asm ("# %cc0: %cc1:" :: ":" (foo), ":" (v), ":" (&w));
--- gcc/testsuite/c-c++-common/toplevel-asm-6.c.jj 2024-11-06
11:13:44.342053423 +0100
+++ gcc/testsuite/c-c++-common/toplevel-asm-6.c 2024-11-06 11:14:27.548444315
+0100
@@ -0,0 +1,9 @@
+/* PR c/41045 */
+/* { dg-do compile } */
+/* { dg-options "-O0" } */
+/* { dg-additional-options "-fPIC" { target fpic } } */
+
+extern int v[42], w;
+void foo (void);
+
+asm ("# %cc0: %cc1:" :: ":" (foo), ":" (v), ":" (&w));
Jakub
