From: Roger Sayle <roger@nextmovesoftware.com>
Date: Fri, 6 Oct 2023 19:11:44 +0000 (+0100)
Subject: i386: Implement doubleword shift left by 1 bit using add+adc.
X-Git-Tag: basepoints/gcc-15~5699
X-Git-Url: http://git.ipfire.org/?a=commitdiff_plain;h=ce658accc7beed19341b6847e6f257f08306a96e;p=thirdparty%2Fgcc.git

i386: Implement doubleword shift left by 1 bit using add+adc.

This patch tweaks the i386 back-end's ix86_split_ashl to implement
doubleword left shifts by 1 bit, using an add followed by an add-with-carry
(i.e. a doubleword x+x) instead of using the x86's shld instruction.
The replacement sequence both requires fewer bytes and is faster on
both Intel and AMD architectures (from Agner Fog's latency tables and
confirmed by my own micro-benchmarking).

For the test case:
__int128 foo(__int128 x) { return x << 1; }

with -O2 we previously generated:

foo:	movq    %rdi, %rax
        movq    %rsi, %rdx
        shldq   $1, %rdi, %rdx
        addq    %rdi, %rax
        ret

with this patch we now generate:

foo:	movq    %rdi, %rax
        movq    %rsi, %rdx
        addq    %rdi, %rax
        adcq    %rsi, %rdx
        ret

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?

2023-10-06  Roger Sayle  <roger@nextmovesoftware.com>

gcc/ChangeLog
	* config/i386/i386-expand.cc (ix86_split_ashl): Split shifts by
	one into add3_cc_overflow_1 followed by add3_carry.
	* config/i386/i386.md (@add<mode>3_cc_overflow_1): Renamed from
	"*add<mode>3_cc_overflow_1" to provide generator function.

gcc/testsuite/ChangeLog
	* gcc.target/i386/ashldi3-2.c: New 32-bit test case.
	* gcc.target/i386/ashlti3-3.c: New 64-bit test case.
---

diff --git a/gcc/config/i386/i386-expand.cc b/gcc/config/i386/i386-expand.cc
index 9a988347200c..425f35318620 100644
--- a/gcc/config/i386/i386-expand.cc
+++ b/gcc/config/i386/i386-expand.cc
@@ -6342,6 +6342,18 @@ ix86_split_ashl (rtx *operands, rtx scratch, machine_mode mode)
 	  if (count > half_width)
 	    ix86_expand_ashl_const (high[0], count - half_width, mode);
 	}
+      else if (count == 1)
+	{
+	  if (!rtx_equal_p (operands[0], operands[1]))
+	    emit_move_insn (operands[0], operands[1]);
+	  rtx x3 = gen_rtx_REG (CCCmode, FLAGS_REG);
+	  rtx x4 = gen_rtx_LTU (mode, x3, const0_rtx);
+	  half_mode = mode == DImode ? SImode : DImode;
+	  emit_insn (gen_add3_cc_overflow_1 (half_mode, low[0],
+					     low[0], low[0]));
+	  emit_insn (gen_add3_carry (half_mode, high[0], high[0], high[0],
+				     x3, x4));
+	}
       else
 	{
 	  gen_shld = mode == DImode ? gen_x86_shld : gen_x86_64_shld;
diff --git a/gcc/config/i386/i386.md b/gcc/config/i386/i386.md
index eef8a0e01ebf..6a5bc16b21cc 100644
--- a/gcc/config/i386/i386.md
+++ b/gcc/config/i386/i386.md
@@ -8864,7 +8864,7 @@
   [(set_attr "type" "alu")
    (set_attr "mode" "<MODE>")])
 
-(define_insn "*add<mode>3_cc_overflow_1"
+(define_insn "@add<mode>3_cc_overflow_1"
   [(set (reg:CCC FLAGS_REG)
 	(compare:CCC
 	    (plus:SWI
diff --git a/gcc/testsuite/gcc.target/i386/ashldi3-2.c b/gcc/testsuite/gcc.target/i386/ashldi3-2.c
new file mode 100644
index 000000000000..053389d1c789
--- /dev/null
+++ b/gcc/testsuite/gcc.target/i386/ashldi3-2.c
@@ -0,0 +1,10 @@
+/* { dg-do compile { target ia32 } } */
+/* { dg-options "-O2 -mno-stv" } */
+
+long long foo(long long x)
+{
+  return x << 1;
+}
+
+/* { dg-final { scan-assembler "adcl" } } */
+/* { dg-final { scan-assembler-not "shldl" } } */
diff --git a/gcc/testsuite/gcc.target/i386/ashlti3-3.c b/gcc/testsuite/gcc.target/i386/ashlti3-3.c
new file mode 100644
index 000000000000..4f14ca0172b6
--- /dev/null
+++ b/gcc/testsuite/gcc.target/i386/ashlti3-3.c
@@ -0,0 +1,10 @@
+/* { dg-do compile { target int128 } } */
+/* { dg-options "-O2" } */
+
+__int128 foo(__int128 x)
+{
+  return x << 1;
+}
+
+/* { dg-final { scan-assembler "adcq" } } */
+/* { dg-final { scan-assembler-not "shldq" } } */