[gcc r14-313] harden-conditionals: detach values before compares

[gcc r14-313] harden-conditionals: detach values before compares

[Alexandre Oliva]

https://gcc.gnu.org/g:f9b9109036110f6a4296b935910db992dc6a34c1

commit r14-313-gf9b9109036110f6a4296b935910db992dc6a34c1
Author: Alexandre Oliva <oliva@adacore.com>
Date:   Thu Apr 27 02:43:48 2023 -0300

    harden-conditionals: detach values before compares
    
    The optimization barriers inserted after compares enable GCC to derive
    information about the values from e.g. the taken paths, or the absence
    of exceptions.  Move them before the original compares, so that the
    reversed compares test copies of the original operands, without
    further optimizations.
    
    
    for  gcc/ChangeLog
    
            * gimple-harden-conditionals.cc (insert_edge_check_and_trap):
            Move detach value calls...
            (pass_harden_conditional_branches::execute): ... here.
            (pass_harden_compares::execute): Detach values before
            compares.
    
    for  gcc/testsuite/ChangeLog
    
            * c-c++-common/torture/harden-cond-comp.c: New.

Diff:
---
 gcc/gimple-harden-conditionals.cc                  | 25 +++++++++++++---------
 .../c-c++-common/torture/harden-cond-comp.c        | 24 +++++++++++++++++++++
 2 files changed, 39 insertions(+), 10 deletions(-)

diff --git a/gcc/gimple-harden-conditionals.cc b/gcc/gimple-harden-conditionals.cc
index 78b8d5692d7..2e5a42e9e71 100644
--- a/gcc/gimple-harden-conditionals.cc
+++ b/gcc/gimple-harden-conditionals.cc
@@ -276,8 +276,8 @@ insert_check_and_trap (location_t loc, gimple_stmt_iterator *gsip,
 }
 
 /* Split edge E, and insert_check_and_trap (see above) in the
-   newly-created block, using detached copies of LHS's and RHS's
-   values (see detach_value above) for the COP compare.  */
+   newly-created block, using already-detached copies of LHS's and
+   RHS's values (see detach_value above) for the COP compare.  */
 
 static inline void
 insert_edge_check_and_trap (location_t loc, edge e,
@@ -301,10 +301,6 @@ insert_edge_check_and_trap (location_t loc, edge e,
 
   gimple_stmt_iterator gsik = gsi_after_labels (chk);
 
-  bool same_p = (lhs == rhs);
-  lhs = detach_value (loc, &gsik, lhs);
-  rhs = same_p ? lhs : detach_value (loc, &gsik, rhs);
-
   insert_check_and_trap (loc, &gsik, flags, cop, lhs, rhs);
 }
 
@@ -366,6 +362,12 @@ pass_harden_conditional_branches::execute (function *fun)
 	/* ??? Can we do better?  */
 	continue;
 
+      /* Detach the values before the compares.  If we do so later,
+	 the compiler may use values inferred from the compares.  */
+      bool same_p = (lhs == rhs);
+      lhs = detach_value (loc, &gsi, lhs);
+      rhs = same_p ? lhs : detach_value (loc, &gsi, rhs);
+
       insert_edge_check_and_trap (loc, EDGE_SUCC (bb, 0), cop, lhs, rhs);
       insert_edge_check_and_trap (loc, EDGE_SUCC (bb, 1), cop, lhs, rhs);
     }
@@ -508,6 +510,13 @@ pass_harden_compares::execute (function *fun)
 
 	  tree rhs = copy_ssa_name (lhs);
 
+	  /* Detach the values before the compares, so that the
+	     compiler infers nothing from them, not even from a
+	     throwing compare that didn't throw.  */
+	  bool same_p = (op1 == op2);
+	  op1 = detach_value (loc, &gsi, op1);
+	  op2 = same_p ? op1 : detach_value (loc, &gsi, op2);
+
 	  gimple_stmt_iterator gsi_split = gsi;
 	  /* Don't separate the original assignment from debug stmts
 	     that might be associated with it, and arrange to split the
@@ -529,10 +538,6 @@ pass_harden_compares::execute (function *fun)
 			 gimple_bb (asgn)->index, nbb->index);
 	    }
 
-	  bool same_p = (op1 == op2);
-	  op1 = detach_value (loc, &gsi_split, op1);
-	  op2 = same_p ? op1 : detach_value (loc, &gsi_split, op2);
-
 	  gassign *asgnck = gimple_build_assign (rhs, cop, op1, op2);
 	  gimple_set_location (asgnck, loc);
 	  gsi_insert_before (&gsi_split, asgnck, GSI_SAME_STMT);
diff --git a/gcc/testsuite/c-c++-common/torture/harden-cond-comp.c b/gcc/testsuite/c-c++-common/torture/harden-cond-comp.c
new file mode 100644
index 00000000000..5aad890a1d3
--- /dev/null
+++ b/gcc/testsuite/c-c++-common/torture/harden-cond-comp.c
@@ -0,0 +1,24 @@
+/* { dg-do compile } */
+/* { dg-options "-fharden-conditional-branches -fharden-compares -fdump-tree-hardcbr -fdump-tree-hardcmp -ffat-lto-objects" } */
+
+int f(int i, int j) {
+  if (i == 0)
+    return j != 0;
+  else
+    return i * j != 0;
+}
+
+/* { dg-final { scan-tree-dump-times "Splitting edge" 2 "hardcbr" } } */
+/* { dg-final { scan-tree-dump-times "Adding reversed compare" 2 "hardcbr" } } */
+/* { dg-final { scan-tree-dump-times "__builtin_trap" 2 "hardcbr" } } */
+
+/* { dg-final { scan-tree-dump-times "Splitting block" 2 "hardcmp" } } */
+/* { dg-final { scan-tree-dump-times "Adding reversed compare" 2 "hardcmp" } } */
+/* { dg-final { scan-tree-dump-times "__builtin_trap" 4 "hardcmp" } } */
+
+/* Check that the optimization barrier is placed before the original compare.  */
+/* { dg-final { scan-tree-dump-times {__asm__[(]"" : "=g" _[0-9]* : "0" i_[0-9]*[(]D[)][)][;][\n][ ]*if [(]i_[0-9]*[(]D[)] == 0[)]} 1 "hardcbr" } } */
+/* { dg-final { scan-tree-dump-times {if [(]_[0-9]* != 0[)]} 2 "hardcbr" } } */
+
+/* { dg-final { scan-tree-dump-times {__asm__[(]"" : "=g" _[0-9]* : "0" j_[0-9]*[(]D[)][)][;][\n][ ]*_[0-9]* = j_[0-9]*[(]D[)] != 0;[\n] *_[0-9]* = _[0-9]* == 0} 1 "hardcmp" } } */
+/* { dg-final { scan-tree-dump-times {__asm__[(]"" : "=g" _[0-9]* : "0" _[0-9]*[)][;][\n][ ]*_[0-9]* = _[0-9]* != 0;[\n] *_[0-9]* = _[0-9]* == 0} 1 "hardcmp" } } */