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* [RFC] expr: don't clear SUBREG_PROMOTED_VAR_P flag for a promoted subreg [target/111466]
@ 2023-09-28 21:43 Vineet Gupta
  2023-09-29  3:17 ` Jeff Law
                   ` (6 more replies)
  0 siblings, 7 replies; 24+ messages in thread
From: Vineet Gupta @ 2023-09-28 21:43 UTC (permalink / raw)
  To: gcc-patches, Robin Dapp
  Cc: kito.cheng, Jeff Law, Palmer Dabbelt, gnu-toolchain, Roger Sayle,
	Jakub Jelinek, Jivan Hakobyan, Vineet Gupta

RISC-V suffers from extraneous sign extensions, despite/given the ABI
guarantee that 32-bit quantities are sign-extended into 64-bit registers,
meaning incoming SI function args need not be explicitly sign extended
(so do SI return values as most ALU insns implicitly sign-extend too.)

Existing REE doesn't seem to handle this well and there are various ideas
floating around to smarten REE about it.

RISC-V also seems to correctly implement middle-end hook PROMOTE_MODE
etc.

Another approach would be to prevent EXPAND from generating the
sign_extend in the first place which this patch tries to do.

The hunk being removed was introduced way back in 1994 as
   5069803972 ("expand_expr, case CONVERT_EXPR .. clear the promotion flag")

This survived full testsuite run for RISC-V rv64gc with surprisingly no
fallouts: test results before/after are exactly same.

|                               | # of unexpected case / # of unique unexpected case
|                               |          gcc |          g++ |     gfortran |
| rv64imafdc_zba_zbb_zbs_zicond/|  264 /    87 |    5 /     2 |   72 /    12 |
|    lp64d/medlow

Granted for something so old to have survived, there must be a valid
reason. Unfortunately the original change didn't have additional
commentary or a test case. That is not to say it can't/won't possibly
break things on other arches/ABIs, hence the RFC for someone to scream
that this is just bonkers, don't do this :-)

I've explicitly CC'ed Jakub and Roger who have last touched subreg
promoted notes in expr.cc for insight and/or screaming ;-)

Thanks to Robin for narrowing this down in an amazing debugging session
@ GNU Cauldron.

```
foo2:
	sext.w	a6,a1             <-- this goes away
	beq	a1,zero,.L4
	li	a5,0
	li	a0,0
.L3:
	addw	a4,a2,a5
	addw	a5,a3,a5
	addw	a0,a4,a0
	bltu	a5,a6,.L3
	ret
.L4:
	li	a0,0
	ret
```

Signed-off-by: Vineet Gupta <vineetg@rivosinc.com>
Co-developed-by: Robin Dapp <rdapp.gcc@gmail.com>
---
 gcc/expr.cc                               |  7 -------
 gcc/testsuite/gcc.target/riscv/pr111466.c | 15 +++++++++++++++
 2 files changed, 15 insertions(+), 7 deletions(-)
 create mode 100644 gcc/testsuite/gcc.target/riscv/pr111466.c

diff --git a/gcc/expr.cc b/gcc/expr.cc
index 308ddc09e631..d259c6e53385 100644
--- a/gcc/expr.cc
+++ b/gcc/expr.cc
@@ -9332,13 +9332,6 @@ expand_expr_real_2 (sepops ops, rtx target, machine_mode tmode,
 	  op0 = expand_expr (treeop0, target, VOIDmode,
 			     modifier);
 
-	  /* If the signedness of the conversion differs and OP0 is
-	     a promoted SUBREG, clear that indication since we now
-	     have to do the proper extension.  */
-	  if (TYPE_UNSIGNED (TREE_TYPE (treeop0)) != unsignedp
-	      && GET_CODE (op0) == SUBREG)
-	    SUBREG_PROMOTED_VAR_P (op0) = 0;
-
 	  return REDUCE_BIT_FIELD (op0);
 	}
 
diff --git a/gcc/testsuite/gcc.target/riscv/pr111466.c b/gcc/testsuite/gcc.target/riscv/pr111466.c
new file mode 100644
index 000000000000..007792466a51
--- /dev/null
+++ b/gcc/testsuite/gcc.target/riscv/pr111466.c
@@ -0,0 +1,15 @@
+/* Simplified varaint of gcc.target/riscv/zba-adduw.c.  */
+
+/* { dg-do compile } */
+/* { dg-options "-march=rv64gc_zba_zbs -mabi=lp64" } */
+/* { dg-skip-if "" { *-*-* } { "-O0" } } */
+
+int foo2(int unused, int n, unsigned y, unsigned delta){
+  int s = 0;
+  unsigned int x = 0;
+  for (;x<n;x +=delta)
+    s += x+y;
+  return s;
+}
+
+/* { dg-final { scan-assembler "\msext\M" } } */
-- 
2.34.1


^ permalink raw reply	[flat|nested] 24+ messages in thread
end of thread, other threads:[~2023-10-17 20:14 UTC | newest]

Thread overview: 24+ messages (download: mbox.gz / follow: Atom feed)
-- links below jump to the message on this page --
2023-09-28 21:43 [RFC] expr: don't clear SUBREG_PROMOTED_VAR_P flag for a promoted subreg [target/111466] Vineet Gupta
2023-09-29  3:17 ` Jeff Law
2023-09-29  3:49   ` Vineet Gupta
2023-09-29 12:14     ` Jeff Law
2023-10-03  1:29       ` Vineet Gupta
2023-10-05 14:56   ` Richard Kenner
2023-10-05 15:04     ` Jeff Law
2023-09-29 10:40 ` Roger Sayle
2023-09-29 13:43   ` Jeff Law
2023-10-04 15:29 ` Jeff Law
2023-10-04 17:59 ` Jeff Law
2023-10-04 18:14   ` Vineet Gupta
2023-10-04 20:11     ` Jeff Law
2023-10-05  4:49 ` Jeff Law
2023-10-05 13:33   ` Robin Dapp
2023-10-05 16:42     ` Jeff Law
2023-10-12  2:37 ` Hans-Peter Nilsson
2023-10-12 16:11   ` Vineet Gupta
2023-10-17  4:07 ` Jeff Law
2023-10-17 18:06   ` Vineet Gupta
2023-10-17 18:07   ` [PATCH] RISC-V/testsuite/pr111466.c: fix expected output to not detect SEXT.W Vineet Gupta
2023-10-17 18:51     ` [PATCH v2] RISC-V/testsuite/pr111466.c: update test and expected output Vineet Gupta
2023-10-17 19:45       ` Jeff Law
2023-10-17 20:14         ` [COMMITTED] " Vineet Gupta

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