[Patch ARM] Fix probe_stack constraint.

[Patch ARM] Fix probe_stack constraint.

[Ramana Radhakrishnan]

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The probe_stack pattern uses r0 as a fixed register. This can cause 
issues if we have auto-increment instructions coming out that have r0 as 
the base register.

Tested with a bootstrap and regression run. richi reports that the 
original issue was fixed in the run. I did consider whether 
probe_stack_range was affected but it all comes back to probe_stack 
pattern so I think we are ok.

I don't have a testcase that seems to provoke this but it seems to be 
default on most distributions so I'm expecting the testcoverage to come 
from there.

Applied to trunk.

Ramana

PR target/82248

* config/arm/arm.md (probe_stack) : Change to using the 'o' constraint.

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diff --git a/gcc/config/arm/arm.md b/gcc/config/arm/arm.md
index 545ee257699..d60c5af551c 100644
--- a/gcc/config/arm/arm.md
+++ b/gcc/config/arm/arm.md
@@ -8612,8 +8612,11 @@
    (set_attr "type" "block")]
 )
 
+;; Since we hard code r0 here use the 'o' constraint to prevent
+;; provoking undefined behaviour in the hardware with putting out
+;; auto-increment operations with potentially r0 as the base register.
 (define_insn "probe_stack"
-  [(set (match_operand:SI 0 "memory_operand" "=m")
+  [(set (match_operand:SI 0 "memory_operand" "=o")
         (unspec:SI [(const_int 0)] UNSPEC_PROBE_STACK))]
   "TARGET_32BIT"
   "str%?\\tr0, %0"