When generating 16-bit code via -m16, the NOP mcount code generation
emits a 5-byte NOP. However, that is neither a valid i8086 instruction
(long NOPs are PentiumPro+), nor would it get decoded as a 5-byte
instruction. It's a 4-byte 'nopw 0(%si)' followed by a zero byte. The
latter causes the following instruction to get misinterpreted as some
form of ADD.
Fix this by emiting a 3-byte no-op 'lea 0(%si)' instead which makes it
compatible with systems lacking long NOP support.
Add a test for this and change the existing one accordingly.
gcc/ChangeLog:
* config/i386/i386.cc (x86_print_call_or_nop): Fix 16-bit NOP
generation.
gcc/testsuite/ChangeLog:
* gcc.target/i386/nop-mcount-m16.c: New test.
* gcc.target/i386/nop-mcount.c: Adapt to test for 5-byte NOP.
Signed-off-by: Mathias Krause <[email protected]>
---
I ran into this bug while trying to abuse '-pg -mnop-mcount' to force
gcc to generate a proper stack frame even for leaf functions to ease
backtracing:
https://lore.kernel.org/kvm/[email protected]/
gcc/config/i386/i386.cc | 12 +++++++++---
gcc/testsuite/gcc.target/i386/nop-mcount-m16.c | 13 +++++++++++++
gcc/testsuite/gcc.target/i386/nop-mcount.c | 6 +++---
3 files changed, 25 insertions(+), 6 deletions(-)
create mode 100644 gcc/testsuite/gcc.target/i386/nop-mcount-m16.c
diff --git a/gcc/config/i386/i386.cc b/gcc/config/i386/i386.cc
index 722c1713a6d3..db43045753bf 100644
--- a/gcc/config/i386/i386.cc
+++ b/gcc/config/i386/i386.cc
@@ -23767,9 +23767,15 @@ x86_print_call_or_nop (FILE *file, const char *target,
const char *label)
{
if (flag_nop_mcount || !strcmp (target, "nop"))
- /* 5 byte nop: nopl 0(%[re]ax,%[re]ax,1) */
- fprintf (file, "%s" ASM_BYTE "0x0f, 0x1f, 0x44, 0x00, 0x00\n",
- label);
+ {
+ if (TARGET_16BIT)
+ /* 3 byte no-op: lea 0(%si), %si */
+ fprintf (file, "%s" ASM_BYTE "0x8d, 0x74, 0x00\n", label);
+ else
+ /* 5 byte nop: nopl 0(%[re]ax,%[re]ax,1) */
+ fprintf (file, "%s" ASM_BYTE "0x0f, 0x1f, 0x44, 0x00, 0x00\n",
+ label);
+ }
else if (!TARGET_PECOFF && flag_pic)
{
gcc_assert (flag_plt);
diff --git a/gcc/testsuite/gcc.target/i386/nop-mcount-m16.c
b/gcc/testsuite/gcc.target/i386/nop-mcount-m16.c
new file mode 100644
index 000000000000..9d019cff5adb
--- /dev/null
+++ b/gcc/testsuite/gcc.target/i386/nop-mcount-m16.c
@@ -0,0 +1,13 @@
+/* Test -mnop-mcount for 16-bit code to generate a 3-byte NOP */
+/* { dg-do compile { target { *-*-linux* && nonpic } } } */
+/* { dg-require-effective-target mfentry } */
+/* { dg-options "-pg -mfentry -mnop-mcount -m16" } */
+/* { dg-final { scan-assembler-not "__fentry__" } } */
+/* { dg-final { scan-assembler-times "0x0f, 0x1f, 0x44, 0x00, 0x00" 0 } } */
+/* { dg-final { scan-assembler-times "0x8d, 0x74, 0x00" 1 } } */
+extern void foobar(char *);
+
+void func(void)
+{
+ foobar ("Hello world\n");
+}
diff --git a/gcc/testsuite/gcc.target/i386/nop-mcount.c
b/gcc/testsuite/gcc.target/i386/nop-mcount.c
index ee3c97f3a57c..35cfef0bc9d7 100644
--- a/gcc/testsuite/gcc.target/i386/nop-mcount.c
+++ b/gcc/testsuite/gcc.target/i386/nop-mcount.c
@@ -1,8 +1,9 @@
/* Test -mnop-mcount */
/* { dg-do compile { target { *-*-linux* && nonpic } } } */
/* { dg-require-effective-target mfentry } */
-/* { dg-options "-Wno-old-style-definition -pg -mfentry -mrecord-mcount
-mnop-mcount" } */
+/* { dg-options "-pg -mfentry -mrecord-mcount -mnop-mcount" } */
/* { dg-final { scan-assembler-not "__fentry__" } } */
+/* { dg-final { scan-assembler-times "0x0f, 0x1f, 0x44, 0x00, 0x00" 3 } } */
/* Origin: Andi Kleen */
extern void foobar(char *);
@@ -18,8 +19,7 @@ void func2(void)
foobar ("Hello world");
}
-void func3(a)
-char *a;
+void func3(char *a)
{
foobar("Hello world");
}
--
2.47.3
