[gcc r14-9864] Generate constant at start of loop, without UB

[gcc r14-9864] Generate constant at start of loop, without UB

[J?rgen Kvalsvik]

https://gcc.gnu.org/g:a2447556a5405d2cde20afc134b90cd1d199ce04

commit r14-9864-ga2447556a5405d2cde20afc134b90cd1d199ce04
Author: Jørgen Kvalsvik <j@lambda.is>
Date:   Mon Apr 8 15:19:55 2024 +0200

    Generate constant at start of loop, without UB
    
    Generating the constants used for recording the edges taken for
    condition coverage would trigger undefined behavior when an expression
    had exactly 64 (== sizeof (1ULL)) conditions, as it would generate the
    constant for the next iteration at the end of the loop body, even if there
    was never a next iteration. By moving the check and constant generation
    to the top of the loop and hoisting the increment flag there is no
    opportunity for UB.
    
            PR middle-end/114627
    
    gcc/ChangeLog:
    
            * tree-profile.cc (instrument_decisions): Generate constant
            at the start of loop.

Diff:
---
 gcc/tree-profile.cc | 20 +++++++++++++++-----
 1 file changed, 15 insertions(+), 5 deletions(-)

diff --git a/gcc/tree-profile.cc b/gcc/tree-profile.cc
index f9d727b3111..b85111624fe 100644
--- a/gcc/tree-profile.cc
+++ b/gcc/tree-profile.cc
@@ -1049,6 +1049,7 @@ instrument_decisions (array_slice<basic_block> expr, size_t condno,
     zerocounter[2] = zero;
 
     unsigned xi = 0;
+    bool increment = false;
     tree rhs = build_int_cst (gcov_type_node, 1ULL << xi);
     for (basic_block current : expr)
     {
@@ -1057,7 +1058,14 @@ instrument_decisions (array_slice<basic_block> expr, size_t condno,
 	    candidates.safe_push (zerocounter);
 	counters prev = resolve_counters (candidates);
 
-	int increment = 0;
+	if (increment)
+	{
+	    xi += 1;
+	    gcc_checking_assert (xi < sizeof (uint64_t) * BITS_PER_UNIT);
+	    rhs = build_int_cst (gcov_type_node, 1ULL << xi);
+	    increment = false;
+	}
+
 	for (edge e : current->succs)
 	{
 	    counters next = prev;
@@ -1072,7 +1080,7 @@ instrument_decisions (array_slice<basic_block> expr, size_t condno,
 		    tree m = build_int_cst (gcov_type_node, masks[2*xi + k]);
 		    next[2] = emit_bitwise_op (e, prev[2], BIT_IOR_EXPR, m);
 		}
-		increment = 1;
+		increment = true;
 	    }
 	    else if (e->flags & EDGE_COMPLEX)
 	    {
@@ -1085,11 +1093,13 @@ instrument_decisions (array_slice<basic_block> expr, size_t condno,
 	    }
 	    table.get_or_insert (e->dest).safe_push (next);
 	}
-	xi += increment;
-	if (increment)
-	    rhs = build_int_cst (gcov_type_node, 1ULL << xi);
     }
 
+    /* Since this is also the return value, the number of conditions, make sure
+       to include the increment of the last basic block.  */
+    if (increment)
+	xi += 1;
+
     gcc_assert (xi == bitmap_count_bits (core));
 
     const tree relaxed = build_int_cst (integer_type_node, MEMMODEL_RELAXED);