https://gcc.gnu.org/g:9b0ae0a8d70603960f3c578d261efd18c02b803f
commit r15-9530-g9b0ae0a8d70603960f3c578d261efd18c02b803f
Author: Ard Biesheuvel <a...@kernel.org>
Date: Thu Apr 10 14:26:40 2025 +0200
i386: Prefer PLT indirection for __fentry__ calls under -fPIC [PR119386]
Commit bde21de1205 ("i386: Honour -mdirect-extern-access when calling
__fentry__") updated the logic that emits mcount() / __fentry__() calls
into function prologues when profiling is enabled, to avoid GOT-based
indirect calls when a direct call would suffice.
There are two problems with that change:
- it relies on -mdirect-extern-access rather than -fno-plt to decide
whether or not a direct [PLT based] call is appropriate;
- for the PLT case, it falls through to x86_print_call_or_nop(), which
does not emit the @PLT suffix, resulting in the wrong relocation to be
used (R_X86_64_PC32 instead of R_X86_64_PLT32)
Fix this by testing flag_plt instead of ix86_direct_extern_access, and
updating x86_print_call_or_nop() to take flag_pic and flag_plt into
account. This also ensures that -mnop-mcount works as expected when
emitting the PLT based profiling calls.
While at it, fix the 32-bit logic as well, and issue a PLT call unless
PLTs are explicitly disabled.
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=119386
Signed-off-by: Ard Biesheuvel <a...@kernel.org>
gcc/ChangeLog:
PR target/119386
* config/i386/i386.cc (x86_print_call_or_nop): Add @PLT suffix
where appropriate.
(x86_function_profiler): Fall through to x86_print_call_or_nop()
for PIC codegen when flag_plt is set.
gcc/testsuite/ChangeLog:
PR target/119386
* gcc.target/i386/pr119386-1.c: New test.
* gcc.target/i386/pr119386-2.c: New test.
Diff:
---
gcc/config/i386/i386.cc | 12 ++++++++++--
gcc/testsuite/gcc.target/i386/pr119386-1.c | 10 ++++++++++
gcc/testsuite/gcc.target/i386/pr119386-2.c | 12 ++++++++++++
3 files changed, 32 insertions(+), 2 deletions(-)
diff --git a/gcc/config/i386/i386.cc b/gcc/config/i386/i386.cc
index b172f716c683..38df84f7db24 100644
--- a/gcc/config/i386/i386.cc
+++ b/gcc/config/i386/i386.cc
@@ -23164,6 +23164,12 @@ x86_print_call_or_nop (FILE *file, const char *target)
if (flag_nop_mcount || !strcmp (target, "nop"))
/* 5 byte nop: nopl 0(%[re]ax,%[re]ax,1) */
fprintf (file, "1:" ASM_BYTE "0x0f, 0x1f, 0x44, 0x00, 0x00\n");
+ else if (!TARGET_PECOFF && flag_pic)
+ {
+ gcc_assert (flag_plt);
+
+ fprintf (file, "1:\tcall\t%s@PLT\n", target);
+ }
else
fprintf (file, "1:\tcall\t%s\n", target);
}
@@ -23327,7 +23333,7 @@ x86_function_profiler (FILE *file, int labelno
ATTRIBUTE_UNUSED)
break;
case CM_SMALL_PIC:
case CM_MEDIUM_PIC:
- if (!ix86_direct_extern_access)
+ if (!flag_plt)
{
if (ASSEMBLER_DIALECT == ASM_INTEL)
fprintf (file, "1:\tcall\t[QWORD PTR %s@GOTPCREL[rip]]\n",
@@ -23358,7 +23364,9 @@ x86_function_profiler (FILE *file, int labelno
ATTRIBUTE_UNUSED)
"\tleal\t%sP%d@GOTOFF(%%ebx), %%" PROFILE_COUNT_REGISTER "\n",
LPREFIX, labelno);
#endif
- if (ASSEMBLER_DIALECT == ASM_INTEL)
+ if (flag_plt)
+ x86_print_call_or_nop (file, mcount_name);
+ else if (ASSEMBLER_DIALECT == ASM_INTEL)
fprintf (file, "1:\tcall\t[DWORD PTR %s@GOT[ebx]]\n", mcount_name);
else
fprintf (file, "1:\tcall\t*%s@GOT(%%ebx)\n", mcount_name);
diff --git a/gcc/testsuite/gcc.target/i386/pr119386-1.c
b/gcc/testsuite/gcc.target/i386/pr119386-1.c
new file mode 100644
index 000000000000..9a0dc64b5b93
--- /dev/null
+++ b/gcc/testsuite/gcc.target/i386/pr119386-1.c
@@ -0,0 +1,10 @@
+/* PR target/119386 */
+/* { dg-do compile { target *-*-linux* } } */
+/* { dg-options "-O2 -fpic -pg" } */
+/* { dg-final { scan-assembler "call\[ \t\]+mcount@PLT" } } */
+
+int
+main ()
+{
+ return 0;
+}
diff --git a/gcc/testsuite/gcc.target/i386/pr119386-2.c
b/gcc/testsuite/gcc.target/i386/pr119386-2.c
new file mode 100644
index 000000000000..3ea978ecfdfd
--- /dev/null
+++ b/gcc/testsuite/gcc.target/i386/pr119386-2.c
@@ -0,0 +1,12 @@
+/* PR target/119386 */
+/* { dg-do compile { target *-*-linux* } } */
+/* { dg-options "-O2 -fpic -fno-plt -pg" } */
+/* { dg-final { scan-assembler "call\[ \t\]+\\*mcount@GOTPCREL\\(" { target {
! ia32 } } } } */
+/* { dg-final { scan-assembler "call\[ \t\]+\\*mcount@GOT\\(" { target ia32 }
} } */
+
+
+int
+main ()
+{
+ return 0;
+}
