[x86 PATCH] PR 106245: Split (x<<31)>>31 as -(x&1) in i386.md

["Roger Sayle" <roger@nextmovesoftware.com>]

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This patch is the backend piece of a solution to PRs 101955 and 106245,
that adds a define_insn_and_split to the i386 backend, to perform sign
extension of a single (least significant) bit using AND $1 then NEG.

Previously, (x<<31)>>31 would be generated as

        sall    $31, %eax       // 3 bytes
        sarl    $31, %eax       // 3 bytes

with this patch the backend now generates:

        andl    $1, %eax        // 3 bytes
        negl    %eax            // 2 bytes

Not only is this smaller in size, but microbenchmarking confirms
that it's a performance win on both Intel and AMD; Intel sees only a
2% improvement (perhaps just a size effect), but AMD sees a 7% win.

This patch has been tested on x86_64-pc-linux-gnu with make bootstrap
and make -k check, both with and without --target_board=unix{-m32}
with no new failures.  Ok for mainline?


2023-10-17  Roger Sayle  <roger@nextmovesoftware.com>

gcc/ChangeLog
        PR middle-end/101955
        PR tree-optimization/106245
        * config/i386/i386.md (*extv<mode>_1_0): New define_insn_and_split.

gcc/testsuite/ChangeLog
        PR middle-end/101955
        PR tree-optimization/106245
        * gcc.target/i386/pr106245-2.c: New test case.
        * gcc.target/i386/pr106245-3.c: New 32-bit test case.
        * gcc.target/i386/pr106245-4.c: New 64-bit test case.
        * gcc.target/i386/pr106245-5.c: Likewise.


Thanks in advance,
Roger
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diff --git a/gcc/config/i386/i386.md b/gcc/config/i386/i386.md
index 2a60df5..b7309be0 100644
--- a/gcc/config/i386/i386.md
+++ b/gcc/config/i386/i386.md
@@ -3414,6 +3414,21 @@
   [(set_attr "type" "imovx")
    (set_attr "mode" "SI")])
 
+;; Split sign-extension of single least significant bit as and x,$1;neg x
+(define_insn_and_split "*extv<mode>_1_0"
+  [(set (match_operand:SWI48 0 "register_operand" "=r")
+	(sign_extract:SWI48 (match_operand:SWI48 1 "register_operand" "0")
+			    (const_int 1)
+			    (const_int 0)))
+   (clobber (reg:CC FLAGS_REG))]
+  ""
+  "#"
+  "&& 1"
+  [(parallel [(set (match_dup 0) (and:SWI48 (match_dup 1) (const_int 1)))
+	      (clobber (reg:CC FLAGS_REG))])
+   (parallel [(set (match_dup 0) (neg:SWI48 (match_dup 0)))
+	      (clobber (reg:CC FLAGS_REG))])])
+
 (define_expand "extzv<mode>"
   [(set (match_operand:SWI248 0 "register_operand")
 	(zero_extract:SWI248 (match_operand:SWI248 1 "register_operand")
diff --git a/gcc/testsuite/gcc.target/i386/pr106245-2.c b/gcc/testsuite/gcc.target/i386/pr106245-2.c
new file mode 100644
index 0000000..47b0d27
--- /dev/null
+++ b/gcc/testsuite/gcc.target/i386/pr106245-2.c
@@ -0,0 +1,10 @@
+/* { dg-do compile } */
+/* { dg-options "-O2" } */
+
+int f(int a)
+{
+    return (a << 31) >> 31;
+}
+
+/* { dg-final { scan-assembler "andl" } } */
+/* { dg-final { scan-assembler "negl" } } */
diff --git a/gcc/testsuite/gcc.target/i386/pr106245-3.c b/gcc/testsuite/gcc.target/i386/pr106245-3.c
new file mode 100644
index 0000000..4ec6342
--- /dev/null
+++ b/gcc/testsuite/gcc.target/i386/pr106245-3.c
@@ -0,0 +1,11 @@
+/* { dg-do compile { target ia32 } } */
+/* { dg-options "-O2" } */
+
+long long f(long long a)
+{
+    return (a << 63) >> 63;
+}
+
+/* { dg-final { scan-assembler "andl" } } */
+/* { dg-final { scan-assembler "negl" } } */
+/* { dg-final { scan-assembler "cltd" } } */
diff --git a/gcc/testsuite/gcc.target/i386/pr106245-4.c b/gcc/testsuite/gcc.target/i386/pr106245-4.c
new file mode 100644
index 0000000..ef77ee5
--- /dev/null
+++ b/gcc/testsuite/gcc.target/i386/pr106245-4.c
@@ -0,0 +1,10 @@
+/* { dg-do compile { target { ! ia32 } } } */
+/* { dg-options "-O2" } */
+
+long long f(long long a)
+{
+    return (a << 63) >> 63;
+}
+
+/* { dg-final { scan-assembler "andl" } } */
+/* { dg-final { scan-assembler "negq" } } */
diff --git a/gcc/testsuite/gcc.target/i386/pr106245-5.c b/gcc/testsuite/gcc.target/i386/pr106245-5.c
new file mode 100644
index 0000000..0351866
--- /dev/null
+++ b/gcc/testsuite/gcc.target/i386/pr106245-5.c
@@ -0,0 +1,11 @@
+/* { dg-do compile { target int128 } } */
+/* { dg-options "-O2" } */
+
+__int128 f(__int128 a)
+{
+  return (a << 127) >> 127;
+}
+
+/* { dg-final { scan-assembler "andl" } } */
+/* { dg-final { scan-assembler "negq" } } */
+/* { dg-final { scan-assembler "cqto" } } */