[v2] PR rtl-optimization/83565: Fix 32-bit rotate on ia64

Message ID 20171224003613.91791-1-jrtc27@jrtc27.com
State New
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  • [v2] PR rtl-optimization/83565: Fix 32-bit rotate on ia64
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Commit Message

Jessica Clarke Dec. 24, 2017, 12:36 a.m.
On ia64, 32-bit rotates are implemented by copying the lower 32 bits of
a register into the upper half, then performing a right shift. However,
depending on the bit pattern in question, this can leave the upper 32
bits as non-zero, despite being only a 32-bit unsigned result. Therefore
add an extra zero_extract to mask these out.

gcc/
	PR rtl-optimization/83565
	* gcc/config/ia64/ia64.md ("*rotrsi3_internal"): Mask out higher 32
	bits from the shift result.
	("*rotlsi3_internal"): Likewise
---

[Resent because git send-email messed about with the headers, adding a
second From: with a different email address, presumably since I ran git
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setting...]

 gcc/config/ia64/ia64.md |   10 ++++++++--
 1 file changed, 8 insertions(+), 2 deletions(-)

--
1.7.10.4

Comments

Jim Wilson Dec. 24, 2017, 5:15 a.m. | #1
On 12/23/2017 04:36 PM, James Clarke wrote:
> 	PR rtl-optimization/83565

> 	* gcc/config/ia64/ia64.md ("*rotrsi3_internal"): Mask out higher 32

> 	bits from the shift result.

> 	("*rotlsi3_internal"): Likewise


This doesn't look right to me.  On ia64, the upper 32-bits of a 32-bit 
value in a 64-bit register are garbage bits.  So there should be no need 
to clear them here after the operation.

Note for instance that lshrsi3 clears the upper 32-bits before shifting 
right, because they are garbage bits.  And note for instance that 
ashlsi3 just shifts left, and doesn't care that it is putting garbage in 
the upper 32-bits.

I think either nonzero bits is broken in the SUBREG case, or ia64 
perhaps should not be setting WORD_REGISTER_OPERATIONS.

Jim

Patch

diff --git a/gcc/config/ia64/ia64.md b/gcc/config/ia64/ia64.md
index b7cd52b..8198b54 100644
--- a/gcc/config/ia64/ia64.md
+++ b/gcc/config/ia64/ia64.md
@@ -3329,7 +3329,10 @@ 
 	(ior:DI (zero_extend:DI (match_dup 1))
 		(ashift:DI (zero_extend:DI (match_dup 1)) (const_int 32))))
    (set (match_dup 3)
-	(lshiftrt:DI (match_dup 3) (match_dup 2)))]
+	(lshiftrt:DI (match_dup 3) (match_dup 2)))
+   (set (match_dup 3)
+	(zero_extract:DI (match_dup 3)
+		(const_int 32) (const_int 0)))]
   "operands[3] = gen_rtx_REG (DImode, REGNO (operands[0]));")

 (define_expand "rotlsi3"
@@ -3358,7 +3361,10 @@ 
 	(ior:DI (zero_extend:DI (match_dup 1))
 		(ashift:DI (zero_extend:DI (match_dup 1)) (const_int 32))))
    (set (match_dup 3)
-	(lshiftrt:DI (match_dup 3) (match_dup 2)))]
+	(lshiftrt:DI (match_dup 3) (match_dup 2)))
+   (set (match_dup 3)
+	(zero_extract:DI (match_dup 3)
+		(const_int 32) (const_int 0)))]
 {
   operands[3] = gen_rtx_REG (DImode, REGNO (operands[0]));
   operands[2] = GEN_INT (32 - INTVAL (operands[2]));