Vineet Gupta <vine...@rivosinc.com> writes:
> On 6/3/25 08:24, Richard Sandiford wrote:
>> I think the issue is that:
>>
>> (insn 9 8 27 2 (parallel [
>> (asm_operands/v ("fsrm %0") ("") 0 [
>> (reg:SI 15 a5 [139])
>> ]
>> [
>> (asm_input:SI ("r") frm-run-1.c:33)
>> ]
>> [] frm-run-1.c:33)
>> (clobber (reg:V4096QI 69 frm))
>> ]) "frm-run-1.c":33:3 -1
>> (nil))
>>
>> is seen as invalidating FRM and so:
>>
>> (insn 27 9 28 2 (set (reg:SI 15 a5 [144])
>> (reg:SI 69 frm)) "frm-run-1.c":43:1 2829 {frrmsi}
>> (nil))
>>
>> is seen as an uninitialised read. I suppose clobbers in inline asms
>> need to be treated as real definitions rather than just kills.
>
> In general or specifically inside of late_combine ?
We can start with the routines that rtl-ssa uses for its dataflow analysis.
Does the patch below help?
Jeff, Eric: any thoughts about this?
Richard
>From 29329926acfb15843f6dc3ac83a74a4e055bb3e6 Mon Sep 17 00:00:00 2001
From: Richard Sandiford <richard.sandif...@arm.com>
Date: Tue, 3 Jun 2025 17:48:12 +0100
Subject: [PATCH] rtlanal: Treat some asm clobbers as rmw accesses, not kills
To: gcc-patches@gcc.gnu.org
This patch treats certain asm clobbers as read-modify-write indicators,
rather than as mere kills. See the comments in the patch for details.
gcc/
* rtlanal.cc asm_clobber_is_read_modify_write_p): New function.
(rtx_properties::try_to_add_pattern): Use it when processing CLOBBERs.
---
gcc/rtlanal.cc | 44 ++++++++++++++++++++++++++++++++++++++++++--
1 file changed, 42 insertions(+), 2 deletions(-)
diff --git a/gcc/rtlanal.cc b/gcc/rtlanal.cc
index 239d6691c4c..b8b563109c1 100644
--- a/gcc/rtlanal.cc
+++ b/gcc/rtlanal.cc
@@ -2072,6 +2072,37 @@ note_uses (rtx *pbody, void (*fun) (rtx *, void *), void
*data)
}
}
+/* Return true if a clobber of X in an asm statement should be treated for
+ dataflow purposes as both a read of X and a write of a meaningful value
+ to X, rather than as a mere kill of X. */
+
+static bool
+asm_clobber_is_read_modify_write_p (rtx x)
+{
+ if (MEM_P (x))
+ /* Asms that clobber memory can read from and write to arbitrary
+ memory locations. */
+ return true;
+
+ if (REG_P (x) && HARD_REGISTER_P (x))
+ /* Some registers are suitable for asm operands. If an asm reads
+ such a register, it should note that in the input operands.
+ If it writes a meaningful value to such a register, it should
+ note that in the output operands. For these registers, a clobber
+ is just a kill. For example, clobbers of argument registers should
+ not keep arbitrary earlier code live.
+
+ However, some fixed registers are not suitable for asm operands.
+ Neither the asm syntax nor the internal representation of asms provide
+ a direct way of indicating that an asm reads from or writes to such
+ registers. The only indication that an asm can give is to clobber
+ the registers. */
+ return !range_in_hard_reg_set_p (operand_reg_set, REGNO (x),
+ REG_NREGS (x));
+
+ return false;
+}
+
/* Try to add a description of REG X to this object, stopping once
the REF_END limit has been reached. FLAGS is a bitmask of
rtx_obj_reference flags that describe the context. */
@@ -2262,8 +2293,17 @@ rtx_properties::try_to_add_pattern (const_rtx pat)
break;
case CLOBBER:
- try_to_add_dest (XEXP (pat, 0), rtx_obj_flags::IS_CLOBBER);
- break;
+ {
+ rtx x = XEXP (pat, 0);
+ if (has_asm && asm_clobber_is_read_modify_write_p (x))
+ {
+ try_to_add_dest (x);
+ try_to_add_src (x);
+ }
+ else
+ try_to_add_dest (x, rtx_obj_flags::IS_CLOBBER);
+ break;
+ }
case SET:
try_to_add_dest (SET_DEST (pat));
--
2.43.0
