[xen master] xen/vpci: Improve code generation in mask_write()

[patchbot@xxxxxxx]

commit 188fa82305e72b725473db9146e20cc9abf7bff3
Author:     Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
AuthorDate: Fri Mar 15 11:31:33 2024 +0000
Commit:     Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
CommitDate: Tue Mar 19 18:29:37 2024 +0000

    xen/vpci: Improve code generation in mask_write()
    
    The use of __clear_bit() forces dmask to be spilled to the stack, and
    interferes with the compiler heuristcs for some upcoming improvements to the
    ffs() code generation.
    
    First, shrink dmask to just the active vectors by making out the upper bits.
    This replaces the "i < msi->vectors" part of the loop condition.
    
    Next, use a simple while() loop with "clear bottom bit" expressed in plane 
C,
    which affords the optimiser a far better understanding of what the loop is
    doing.
    
    Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
    Reviewed-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>
---
 xen/drivers/vpci/msi.c | 10 ++++++----
 1 file changed, 6 insertions(+), 4 deletions(-)

diff --git a/xen/drivers/vpci/msi.c b/xen/drivers/vpci/msi.c
index d3aa5df089..30adcf7df0 100644
--- a/xen/drivers/vpci/msi.c
+++ b/xen/drivers/vpci/msi.c
@@ -169,13 +169,15 @@ static void cf_check mask_write(
 
     if ( msi->enabled )
     {
-        unsigned int i;
+        /* Skip changes to vectors which aren't enabled. */
+        dmask &= (~0U >> (32 - msi->vectors));
 
-        for ( i = ffs(dmask) - 1; dmask && i < msi->vectors;
-              i = ffs(dmask) - 1 )
+        while ( dmask )
         {
+            unsigned int i = ffs(dmask) - 1;
+
             vpci_msi_arch_mask(msi, pdev, i, (val >> i) & 1);
-            __clear_bit(i, &dmask);
+            dmask &= (dmask - 1);
         }
     }
 
--
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