[PATCH] i386: Prevent unwanted combine from LZCNT to BSR [PR101175]

Uros Bizjak ubizjak@gmail.com
Wed Jun 23 10:53:28 GMT 2021


The current RTX pattern for BSR allows combine pass to convert LZCNT insn
to BSR. Note that the LZCNT has a defined behavior to return the operand
size when operand is zero, where BSR has not.

Add a BSR specific setting of zero-flag to RTX pattern of BSR insn
in order to avoid matching unwanted combinations.

2021-06-23  Uroš Bizjak  <ubizjak@gmail.com>

gcc/
    PR target/101175
    * config/i386/i386.md (bsr_rex64): Add zero-flag setting RTX.
    (bsr): Ditto.
    (*bsrhi): Remove.
    (clz<mode>2): Update RTX pattern for additions.

gcc/testsuite/

    PR target/101175
    * gcc.target/i386/pr101175.c: New test.

Bootstrapped and regression tested on x86_64-linux-gnu {,-m32}.

Pushed to master, will be backported to release branches.

Uros.
-------------- next part --------------
diff --git a/gcc/config/i386/i386.md b/gcc/config/i386/i386.md
index 700c158a432..4e242105719 100644
--- a/gcc/config/i386/i386.md
+++ b/gcc/config/i386/i386.md
@@ -14533,10 +14533,12 @@ (define_insn "*ctzsi2_zext_falsedep"
    (set_attr "mode" "SI")])
 
 (define_insn "bsr_rex64"
-  [(set (match_operand:DI 0 "register_operand" "=r")
+  [(set (reg:CCZ FLAGS_REG)
+	(compare:CCZ (match_operand:DI 1 "nonimmediate_operand" "rm")
+		     (const_int 0)))
+   (set (match_operand:DI 0 "register_operand" "=r")
 	(minus:DI (const_int 63)
-		  (clz:DI (match_operand:DI 1 "nonimmediate_operand" "rm"))))
-   (clobber (reg:CC FLAGS_REG))]
+		  (clz:DI (match_dup 1))))]
   "TARGET_64BIT"
   "bsr{q}\t{%1, %0|%0, %1}"
   [(set_attr "type" "alu1")
@@ -14545,10 +14547,12 @@ (define_insn "bsr_rex64"
    (set_attr "mode" "DI")])
 
 (define_insn "bsr"
-  [(set (match_operand:SI 0 "register_operand" "=r")
+  [(set (reg:CCZ FLAGS_REG)
+	(compare:CCZ (match_operand:SI 1 "nonimmediate_operand" "rm")
+		     (const_int 0)))
+   (set (match_operand:SI 0 "register_operand" "=r")
 	(minus:SI (const_int 31)
-		  (clz:SI (match_operand:SI 1 "nonimmediate_operand" "rm"))))
-   (clobber (reg:CC FLAGS_REG))]
+		  (clz:SI (match_dup 1))))]
   ""
   "bsr{l}\t{%1, %0|%0, %1}"
   [(set_attr "type" "alu1")
@@ -14556,25 +14560,15 @@ (define_insn "bsr"
    (set_attr "znver1_decode" "vector")
    (set_attr "mode" "SI")])
 
-(define_insn "*bsrhi"
-  [(set (match_operand:HI 0 "register_operand" "=r")
-	(minus:HI (const_int 15)
-		  (clz:HI (match_operand:HI 1 "nonimmediate_operand" "rm"))))
-   (clobber (reg:CC FLAGS_REG))]
-  ""
-  "bsr{w}\t{%1, %0|%0, %1}"
-  [(set_attr "type" "alu1")
-   (set_attr "prefix_0f" "1")
-   (set_attr "znver1_decode" "vector")
-   (set_attr "mode" "HI")])
-
 (define_expand "clz<mode>2"
   [(parallel
-     [(set (match_operand:SWI48 0 "register_operand")
+     [(set (reg:CCZ FLAGS_REG)
+	(compare:CCZ (match_operand:SWI48 1 "nonimmediate_operand" "rm")
+		     (const_int 0)))
+      (set (match_operand:SWI48 0 "register_operand")
 	   (minus:SWI48
 	     (match_dup 2)
-	     (clz:SWI48 (match_operand:SWI48 1 "nonimmediate_operand"))))
-      (clobber (reg:CC FLAGS_REG))])
+	     (clz:SWI48 (match_dup 1))))])
    (parallel
      [(set (match_dup 0) (xor:SWI48 (match_dup 0) (match_dup 2)))
       (clobber (reg:CC FLAGS_REG))])]
diff --git a/gcc/testsuite/gcc.target/i386/pr101175.c b/gcc/testsuite/gcc.target/i386/pr101175.c
new file mode 100644
index 00000000000..ed7a08110a5
--- /dev/null
+++ b/gcc/testsuite/gcc.target/i386/pr101175.c
@@ -0,0 +1,28 @@
+/* { dg-do run } */
+/* { dg-options "-O2 -mlzcnt" } */
+/* { dg-require-effective-target lzcnt } */
+
+#include "lzcnt-check.h"
+
+static int
+foo (unsigned int v)
+{
+  return v ? __builtin_clz (v) : 32;
+}
+
+/* returns -1 if x == 0 */
+int
+__attribute__ ((noinline, noclone))
+bar (unsigned int x)
+{
+  return 31 - foo (x);
+}
+
+static void
+lzcnt_test ()
+{
+  int r = bar (0);
+
+  if (r != -1)
+    abort ();
+}


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