On Tue, Mar 16, 2021 at 11:55:01AM +0100, Uros Bizjak wrote:
> Maybe we could simply emit a special form of a ASHIFT pattern, tagged
> with some unspec (similar to e.g. divmod<mode>4_1), and teach
> ix86_split_lea_for_addr to emit it instead? Peephole pass is so late
> in the pass sequence that we won't lose anything. We only need one
> additional SWI48mode ASHIFT pattern with a const123_operand immediate.
Ok. Any reason not to use just MULT for that and split it back into
ASHIFT during the split pass that follows shortly after peephole2?
2021-03-16 Jakub Jelinek <ja...@redhat.com>
PR target/99600
* config/i386/i386-expand.c (ix86_split_lea_for_addr): Emit a MULT
rather than ASHIFT.
* config/i386/i386.md (mult by 1248 into ashift): New splitter.
* gcc.target/i386/pr99600.c: New test.
--- gcc/config/i386/i386-expand.c.jj 2021-03-16 11:16:08.487860451 +0100
+++ gcc/config/i386/i386-expand.c 2021-03-16 12:26:20.331083409 +0100
@@ -1348,9 +1348,10 @@ ix86_split_lea_for_addr (rtx_insn *insn,
if (regno0 != regno2)
emit_insn (gen_rtx_SET (target, parts.index));
- /* Use shift for scaling. */
- ix86_emit_binop (ASHIFT, mode, target,
- GEN_INT (exact_log2 (parts.scale)));
+ /* Use shift for scaling, but emit it as MULT instead
+ to avoid it being immediately peephole2 optimized back
+ into lea. */
+ ix86_emit_binop (MULT, mode, target, GEN_INT (parts.scale));
if (parts.base)
ix86_emit_binop (PLUS, mode, target, parts.base);
--- gcc/config/i386/i386.md.jj 2021-03-16 00:21:12.192422264 +0100
+++ gcc/config/i386/i386.md 2021-03-16 12:41:27.384022356 +0100
@@ -5219,6 +5219,18 @@ (define_peephole2
DONE;
})
+
+;; ix86_split_lea_for_addr emits the shifts as MULT to avoid it from being
+;; peephole2 optimized back into a lea. Split that into the shift during
+;; the following split pass.
+(define_split
+ [(set (match_operand:SWI48 0 "general_reg_operand")
+ (mult:SWI48 (match_dup 0) (match_operand:SWI48 1 "const1248_operand")))
+ (clobber (reg:CC FLAGS_REG))]
+ "reload_completed"
+ [(parallel [(set (match_dup 0) (ashift:SWI48 (match_dup 0) (match_dup 1)))
+ (clobber (reg:CC FLAGS_REG))])]
+ "operands[1] = GEN_INT (exact_log2 (INTVAL (operands[1])));")
;; Add instructions
--- gcc/testsuite/gcc.target/i386/pr99600.c.jj 2021-03-16 12:26:50.610747515
+0100
+++ gcc/testsuite/gcc.target/i386/pr99600.c 2021-03-16 12:26:50.610747515
+0100
@@ -0,0 +1,16 @@
+/* PR target/99600 */
+/* { dg-do compile } */
+/* { dg-options "-O2 -march=atom" } */
+
+char a, b;
+long c;
+
+long
+foo (void)
+{
+ if (a)
+ c = b == 1 ? 1 << 3 : 1 << 2;
+ else
+ c = 0;
+ return 0;
+}
Jakub
