From: Rhys Weatherley <rweather@southern-storm.com.au>
Date: Thu, 20 May 2004 09:53:37 +0000 (+0000)
Subject: Treat dead blocks as empty when peepholing branches to the next block.
X-Git-Tag: r.0.0.4~99
X-Git-Url: https://git.unchartedbackwaters.co.uk/w/?a=commitdiff_plain;h=00351eaf603151ab3d36c0c514482da646e84a9b;p=francis%2Flibjit.git

Treat dead blocks as empty when peepholing branches to the next block.
---

diff --git a/ChangeLog b/ChangeLog
index 871c03b..d1d964b 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -10,6 +10,9 @@
 	branches to branches before live variable analysis, so that
 	the back ends don't need to worry about jump threading.
 
+	* jit/jit-block.c, jit/jit-live.c: treat dead blocks as empty
+	when peepholing branches to the next block.
+
 2004-05-15  Rhys Weatherley  <rweather@southern-storm.com.au>
 
 	* tools/gen-apply.c: fix a macro generation bug for Win32 systems.
diff --git a/jit/jit-block.c b/jit/jit-block.c
index 8ef0a89..abc5b13 100644
--- a/jit/jit-block.c
+++ b/jit/jit-block.c
@@ -383,6 +383,25 @@ int jit_block_ends_in_dead(jit_block_t block)
 	return block->ends_in_dead;
 }
 
+/*
+ * Determine if a block is empty or is never entered.
+ */
+static int block_is_empty_or_dead(jit_block_t block)
+{
+	if(block->first_insn > block->last_insn)
+	{
+		return 1;
+	}
+	else if(!(block->entered_via_top) && !(block->entered_via_branch))
+	{
+		return 1;
+	}
+	else
+	{
+		return 0;
+	}
+}
+
 /*
  * Determine if the next block after "block" has "label".
  */
@@ -396,29 +415,33 @@ static int block_branches_to_next(jit_block_t block, jit_label_t label)
 		{
 			return 1;
 		}
-		if(block->first_insn < block->last_insn)
-		{
-			/* This block contains more than one instruction, so the
-			   first cannot be an unconditional branch */
-			break;
-		}
-		else if(block->first_insn == block->last_insn)
+		if(!block_is_empty_or_dead(block))
 		{
-			insn = block->func->builder->insns[block->first_insn];
-			if(insn->opcode == JIT_OP_BR)
+			if(block->first_insn < block->last_insn)
 			{
-				/* If the instruction branches to its next block,
-				   then it is equivalent to an empty block.  If it
-				   does not, then we have to stop scanning here */
-				if(!block_branches_to_next(block, (jit_label_t)(insn->dest)))
-				{
-					return 0;
-				}
+				/* This block contains more than one instruction, so the
+				   first cannot be an unconditional branch */
+				break;
 			}
 			else
 			{
-				/* The block does not contain an unconditional branch */
-				break;
+				insn = block->func->builder->insns[block->first_insn];
+				if(insn->opcode == JIT_OP_BR)
+				{
+					/* If the instruction branches to its next block,
+					   then it is equivalent to an empty block.  If it
+					   does not, then we have to stop scanning here */
+					if(!block_branches_to_next
+							(block, (jit_label_t)(insn->dest)))
+					{
+						return 0;
+					}
+				}
+				else
+				{
+					/* The block does not contain an unconditional branch */
+					break;
+				}
 			}
 		}
 		block = block->next;
@@ -449,7 +472,7 @@ void _jit_block_peephole_branch(jit_block_t block)
 	while(label != block->label && count > 0)
 	{
 		new_block = jit_block_from_label(block->func, label);
-		while(new_block != 0 && new_block->first_insn > new_block->last_insn)
+		while(new_block != 0 && block_is_empty_or_dead(new_block))
 		{
 			/* Skip past empty blocks */
 			new_block = new_block->next;
diff --git a/jit/jit-live.c b/jit/jit-live.c
index 5193313..6ee52b2 100644
--- a/jit/jit-live.c
+++ b/jit/jit-live.c
@@ -199,8 +199,19 @@ void _jit_function_compute_liveness(jit_function_t func)
 	jit_block_t block = func->builder->first_block;
 	while(block != 0)
 	{
+		/* If the block is never entered, then replace it with empty */
+		if(!(block->entered_via_top) && !(block->entered_via_branch))
+		{
+			block->last_insn = block->first_insn - 1;
+		}
+
+		/* Perform peephole optimization on branches to branches */
 		_jit_block_peephole_branch(block);
+
+		/* Compute the liveness flags for the block */
 		compute_liveness_for_block(block);
+
+		/* Move on to the next block in the function */
 		block = block->next;
 	}
 }