[PATCH] D139637: [AArch64][SVE][ISel] Combine dup of load to replicating load

[Dave Green via Phabricator via llvm-commits]

dmgreen added inline comments.


================
Comment at: llvm/test/tools/llvm-mca/AArch64/Neoverse/N2-sve-instructions.s:4475
 # CHECK-NEXT:  1      6     0.50    *                   ld1h	{ z5.h }, p3/z, [x17, x16, lsl #1]
-# CHECK-NEXT:  1      6     0.33    *             U     ld1rb	{ z0.b }, p0/z, [x0]
-# CHECK-NEXT:  1      6     0.33    *             U     ld1rb	{ z0.d }, p0/z, [x0]
-# CHECK-NEXT:  1      6     0.33    *             U     ld1rb	{ z0.h }, p0/z, [x0]
-# CHECK-NEXT:  1      6     0.33    *             U     ld1rb	{ z0.s }, p0/z, [x0]
-# CHECK-NEXT:  1      6     0.33    *             U     ld1rb	{ z31.b }, p7/z, [sp, #63]
-# CHECK-NEXT:  1      6     0.33    *             U     ld1rb	{ z31.d }, p7/z, [sp, #63]
-# CHECK-NEXT:  1      6     0.33    *             U     ld1rb	{ z31.h }, p7/z, [sp, #63]
-# CHECK-NEXT:  1      6     0.33    *             U     ld1rb	{ z31.s }, p7/z, [sp, #63]
-# CHECK-NEXT:  1      6     0.33    *             U     ld1rd	{ z0.d }, p0/z, [x0]
-# CHECK-NEXT:  1      6     0.33    *             U     ld1rd	{ z31.d }, p7/z, [sp, #504]
-# CHECK-NEXT:  1      6     0.33    *             U     ld1rh	{ z0.d }, p0/z, [x0]
-# CHECK-NEXT:  1      6     0.33    *             U     ld1rh	{ z0.h }, p0/z, [x0]
-# CHECK-NEXT:  1      6     0.33    *             U     ld1rh	{ z0.s }, p0/z, [x0]
-# CHECK-NEXT:  1      6     0.33    *             U     ld1rh	{ z31.d }, p7/z, [sp, #126]
-# CHECK-NEXT:  1      6     0.33    *             U     ld1rh	{ z31.h }, p7/z, [sp, #126]
-# CHECK-NEXT:  1      6     0.33    *             U     ld1rh	{ z31.s }, p7/z, [sp, #126]
+# CHECK-NEXT:  1      6     0.33    *                   ld1rb	{ z0.b }, p0/z, [x0]
+# CHECK-NEXT:  1      6     0.33    *                   ld1rb	{ z0.d }, p0/z, [x0]
----------------
Allen wrote:
> Excuse me, I'm curious why the above changes affect the **SideEffects attribute** here. Would you give me some guidance? Thank you.
The way that hasSideEffects for an instruction works is that:
 - It defaults to true.
 - It gets set to false if there is a tablegen pattern that generated that instruction (I believe it might needs to be a single instruction generated by the pattern).
 - It can be overridden with `let hasSideEffects=1/0` on the instruction.

If you are looking to remove the side effects flags, and the instruction doesn't have a pattern, adding hasSideEffects=0 is usually a good way to go. Removing the side effects can help improve scheduling freedom and other code motion in the backend, so is usually good to do so long as the instruction doesn't do anything odd.


Repository:
  rG LLVM Github Monorepo

CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D139637/new/

https://reviews.llvm.org/D139637