[PATCH, GCC/LRA] Fix PR78617: Fix conflict detection in rematerialization

[Thomas Preudhomme]

Hi,

When considering a candidate for rematerialization, LRA verifies if the 
candidate clobbers a live register before going forward with the 
rematerialization (see code starting with comment "Check clobbers do not kill 
something living."). To do this check, the set of live registers at any given 
instruction needs to be maintained. This is done by initializing the set of live 
registers when starting the forward scan of instruction in a basic block and 
updating the set by looking at REG_DEAD notes and destination register at the 
end of an iteration of the scan loop.

However the initialization suffers from 2 issues:

1) it is done from the live out set rather than live in (uses df_get_live_out 
(bb))
2) it ignores pseudo registers that have already been allocated a hard register 
(uses REG_SET_TO_HARD_REG_SET that only looks at hard register and does not look 
at reg_renumber for pseudo registers)

This patch changes the code to use df_get_live_in (bb) to initialize the 
live_hard_regs variable using a loop to check reg_renumber for pseudo registers. 
Please let me know if there is a macro to do that, I failed to find one.

ChangeLog entries are as follow:

gcc/testsuite/ChangeLog:

2016-12-01  Thomas Preud'homme  <thomas.preudho...@arm.com>

        PR rtl-optimization/78617
        * gcc.c-torture/execute/pr78617.c: New test.


gcc/ChangeLog:

2016-12-01  Thomas Preud'homme  <thomas.preudho...@arm.com>

        PR rtl-optimization/78617
        * lra-remat.c (do_remat): Initialize live_hard_regs from live in
        registers, also setting hard registers mapped to pseudo registers.


Note however that as explained in the problem report, the testcase does not 
trigger the bug on GCC 7 due to better optimization before LRA rematerialization 
is reached.

Testing: testsuite shows no regression when run using:
 + an arm-none-eabi GCC cross-compiler targeting Cortex-M0 and Cortex-M3
 + a bootstrapped arm-linux-gnueabihf GCC native compiler
 + a bootstrapped x86_64-linux-gnu GCC native compiler

Is this ok for stage3?

Best regards,

Thomas
diff --git a/gcc/lra-remat.c b/gcc/lra-remat.c
index f01c6644c428fd9b5efdf6cc98788e5f6fadba62..cdd7057f602098d33ec3acfdaaac66556640bd82 100644
--- a/gcc/lra-remat.c
+++ b/gcc/lra-remat.c
@@ -1047,6 +1047,7 @@ update_scratch_ops (rtx_insn *remat_insn)
 static bool
 do_remat (void)
 {
+  unsigned regno;
   rtx_insn *insn;
   basic_block bb;
   bitmap_head avail_cands;
@@ -1054,12 +1055,21 @@ do_remat (void)
   bool changed_p = false;
   /* Living hard regs and hard registers of living pseudos.  */
   HARD_REG_SET live_hard_regs;
+  bitmap_iterator bi;
 
   bitmap_initialize (&avail_cands, &reg_obstack);
   bitmap_initialize (&active_cands, &reg_obstack);
   FOR_EACH_BB_FN (bb, cfun)
     {
-      REG_SET_TO_HARD_REG_SET (live_hard_regs, df_get_live_out (bb));
+      CLEAR_HARD_REG_SET (live_hard_regs);
+      EXECUTE_IF_SET_IN_BITMAP (df_get_live_in (bb), 0, regno, bi)
+	{
+	  int hard_regno = regno < FIRST_PSEUDO_REGISTER
+			   ? regno
+			   : reg_renumber[regno];
+	  if (hard_regno >= 0)
+	    SET_HARD_REG_BIT (live_hard_regs, hard_regno);
+	}
       bitmap_and (&avail_cands, &get_remat_bb_data (bb)->avin_cands,
 		  &get_remat_bb_data (bb)->livein_cands);
       /* Activating insns are always in the same block as their corresponding
diff --git a/gcc/testsuite/gcc.c-torture/execute/pr78617.c b/gcc/testsuite/gcc.c-torture/execute/pr78617.c
new file mode 100644
index 0000000000000000000000000000000000000000..89c4f6dea8cb507b963f91debb94cbe16eb1db90
--- /dev/null
+++ b/gcc/testsuite/gcc.c-torture/execute/pr78617.c
@@ -0,0 +1,25 @@
+int a = 0;
+int d = 1;
+int f = 1;
+
+int fn1() {
+  return a || 1 >> a;
+}
+
+int fn2(int p1, int p2) {
+  return p2 >= 2 ? p1 : p1 >> 1;
+}
+
+int fn3(int p1) {
+  return d ^ p1;
+}
+
+int fn4(int p1, int p2) {
+  return fn3(!d > fn2((f = fn1() - 1000) || p2, p1));
+}
+
+int main() {
+  if (fn4(0, 0) != 1)
+    __builtin_abort ();
+  return 0;
+}