[LangRef] Clarify in vscale_range that vscale is a power-of-two without the attribute

[lukel97]

Previously vscale_range used to add the constraint that vscale is a
power-of-two, but after #183080 it's already a power-of-two to begin with.

This clarifies the sentence about assumptions when there is no attribute

[llvmbot]

@llvm/pr-subscribers-llvm-ir

Author: Luke Lau (lukel97)

Changes

Previously vscale_range used to add the constraint that vscale is a
power-of-two, but after #183080 it's already a power-of-two to begin with.

This drops the extra mention of power-of-two to clarify it's not a new
constraint now.

---

Full diff: https://github.com/llvm/llvm-project/pull/183689.diff

1 Files Affected:

• (modified) llvm/docs/LangRef.rst (+6-6)

diff --git a/llvm/docs/LangRef.rst b/llvm/docs/LangRef.rst
index 2f5ba6c22ddab..4142e496cdbe2 100644
--- a/llvm/docs/LangRef.rst
+++ b/llvm/docs/LangRef.rst
@@ -2771,12 +2771,12 @@ For example:
     than `UINT_MAX`.  It's unspecified which threshold will be used when
     duplicate definitions are linked together with differing values.
 ``vscale_range(<min>[, <max>])``
-    This function attribute indicates `vscale` is a power-of-two within a
-    specified range. `min` must be a power-of-two that is greater than 0. When
-    specified, `max` must be a power-of-two greater-than-or-equal to `min` or 0
-    to signify an unbounded maximum. The syntax `vscale_range(<val>)` can be
-    used to set both `min` and `max` to the same value. Functions that don't
-    include this attribute make no assumptions about the value of `vscale`.
+    This function attribute indicates `vscale` is within a specified
+    range. `min` must be a power-of-two that is greater than 0. When specified,
+    `max` must be a power-of-two greater-than-or-equal to `min` or 0 to signify
+    an unbounded maximum. The syntax `vscale_range(<val>)` can be used to set
+    both `min` and `max` to the same value. Functions that don't include this
+    attribute make no assumptions about the range of `vscale`.
 ``nooutline``
     This attribute indicates that outlining passes should not modify the
     function.

[paulwalker-arm]

Please can we keep the explicitness here? I don't see a problem with being more verbose. Also, while there's an option, however unlikely, of walking back the "vscale is a power-of-two" decision, this would not apply to vscale_range.

[lukel97]

Please can we keep the explicitness here? I don't see a problem with being more verbose. Also, while there's an option, however unlikely, of walking back the "vscale is a power-of-two" decision, this would not apply to vscale_range.

My concern is that we have places where we checked vscale_range to get the power-of-2 guarantee, and I don't want developers to read the vscale_range part in the LangRef and think it's still needed.

It's not a major concern though, dropped from this PR. I've still kept the update to the "make no assumptions about the value of vscale" part though since I don't think that's accurate anymore