[LangRef] Cap maximum value of vscale at 2^31-1.

[fhahn]

Cap the maximum value of vscale at 2^31-1. This should exceed what is available on actual hardware by far but should be enough to ensure that computing the runtime VF vscale x V generated by LoopVectorize won't wrap when using i64 index types, as V cannot exceed 2^32. LoopVectorize already assumes it won't wrap, this change tries to back up the assumption in LangRef.

[llvmbot]

@llvm/pr-subscribers-llvm-ir

Author: Florian Hahn (fhahn)

Changes

Cap the maximum value of vscale at 2^31-1. This should exceed what is available on actual hardware by far but should be enough to ensure that computing the runtime VF vscale x V generated by LoopVectorize won't wrap when using i64 index types, as V cannot exceed 2^32. LoopVectorize already assumes it won't wrap, this change tries to back up the assumption in LangRef.

---

Full diff: https://github.com/llvm/llvm-project/pull/144607.diff

1 Files Affected:

• (modified) llvm/docs/LangRef.rst (+2-1)

diff --git a/llvm/docs/LangRef.rst b/llvm/docs/LangRef.rst
index cc72a37f68599..b876d0bdbc29e 100644
--- a/llvm/docs/LangRef.rst
+++ b/llvm/docs/LangRef.rst
@@ -4446,6 +4446,7 @@ of elements; vscale is a positive integer that is unknown at compile time
 and the same hardware-dependent constant for all scalable vectors at run
 time. The size of a specific scalable vector type is thus constant within
 IR, even if the exact size in bytes cannot be determined until run time.
+vscale can be at most 2^31-1.
 
 :Examples:
 
@@ -30399,7 +30400,7 @@ Semantics:
 """"""""""
 
 ``vscale`` is a positive value that is constant throughout program
-execution, but is unknown at compile time.
+execution, but is unknown at compile time. The returned value can be at most 2^31-1.
 If the result value does not fit in the result type, then the result is
 a :ref:`poison value <poisonvalues>`.
 

[preames]

LGTM

[efriedma-quic]

runtime VF vscale x V generated by LoopVectorize won't wrap when using i64 index types

What about 32-bit targets?

(Not that I'm objecting to this... you'd need to be doing something really unusual with your target to end up with vscales above 1000.)

[topperc]

runtime VF vscale x V generated by LoopVectorize won't wrap when using i64 index types

What about 32-bit targets?

(Not that I'm objecting to this... you'd need to be doing something really unusual with your target to end up with vscales above 1000.)

If it helps, the maximum vscale for RISC-V is currently 1024. If we ever fix some other issues that prevent us from supporting the smallest vector length of 32, then the maximum vscale will be 2048.