[PATCH 3/5] objtool: Prepare for scratch regs

[Peter Zijlstra]

Introduce __save_reg() which allows using the
!arch_callee_saved_reg()s and make sure they're wiped after every
stack op so they don't linger, allow for a single op exception so they
can be used on the very next stack-op.

Signed-off-by: Peter Zijlstra (Intel) <pet...@infradead.org>
---
 tools/objtool/check.c |   27 ++++++++++++++++++++++++---
 1 file changed, 24 insertions(+), 3 deletions(-)

--- a/tools/objtool/check.c
+++ b/tools/objtool/check.c
@@ -1775,15 +1775,20 @@ static int update_cfi_state_regs(struct
        return 0;
 }
 
-static void save_reg(struct cfi_state *cfi, unsigned char reg, int base, int 
offset)
+static void __save_reg(struct cfi_state *cfi, unsigned char reg, int base, int 
offset)
 {
-       if (arch_callee_saved_reg(reg) &&
-           cfi->regs[reg].base == CFI_UNDEFINED) {
+       if (cfi->regs[reg].base == CFI_UNDEFINED) {
                cfi->regs[reg].base = base;
                cfi->regs[reg].offset = offset;
        }
 }
 
+static void save_reg(struct cfi_state *cfi, unsigned char reg, int base, int 
offset)
+{
+       if (arch_callee_saved_reg(reg))
+               __save_reg(cfi, reg, base, offset);
+}
+
 static void restore_reg(struct cfi_state *cfi, unsigned char reg)
 {
        cfi->regs[reg].base = initial_func_cfi.regs[reg].base;
@@ -1848,6 +1853,7 @@ static int update_cfi_state(struct instr
 {
        struct cfi_reg *cfa = &cfi->cfa;
        struct cfi_reg *regs = cfi->regs;
+       bool skip_wipe = false;
 
        /* stack operations don't make sense with an undefined CFA */
        if (cfa->base == CFI_UNDEFINED) {
@@ -2192,6 +2198,21 @@ static int update_cfi_state(struct instr
                return -1;
        }
 
+       /*
+        * Only callee saved registers are preserved; the rest is scratch space
+        * preserved at most one instruction.
+        */
+       if (!skip_wipe) {
+               int i;
+
+               for (i = 0; i < CFI_NUM_REGS; i++) {
+                       if (arch_callee_saved_reg(i))
+                               continue;
+
+                       restore_reg(cfi, i);
+               }
+       }
+
        return 0;
 }