[llvm] Provide intrinsics for speculative loads (PR #179642)

[Florian Hahn via llvm-commits]

================
@@ -0,0 +1,75 @@
+; NOTE: Assertions have been autogenerated by utils/update_test_checks.py
+; RUN: opt -mtriple=aarch64-unknown-linux-gnu -passes=pre-isel-intrinsic-lowering -S < %s | FileCheck %s
+
+; Test that @llvm.can.load.speculatively is lowered to an alignment check
+; for power-of-2 sizes <= 16 bytes on AArch64, and returns false for larger sizes.
+; The 16-byte limit ensures correctness with MTE (memory tagging).
+; Note: non-power-of-2 constant sizes are rejected by the verifier.
+
+define i1 @can_load_speculatively_16(ptr %ptr) {
+; CHECK-LABEL: @can_load_speculatively_16(
+; CHECK-NEXT:    [[TMP1:%.*]] = ptrtoint ptr [[PTR:%.*]] to i64
+; CHECK-NEXT:    [[TMP2:%.*]] = and i64 [[TMP1]], 15
+; CHECK-NEXT:    [[TMP3:%.*]] = icmp eq i64 [[TMP2]], 0
+; CHECK-NEXT:    ret i1 [[TMP3]]
+;
+  %can_load = call i1 @llvm.can.load.speculatively.p0(ptr %ptr, i64 16)
+  ret i1 %can_load
+}
+
+; Size > 16 - returns false (may cross MTE tag granule boundary)
+define i1 @can_load_speculatively_32(ptr %ptr) {
+; CHECK-LABEL: @can_load_speculatively_32(
+; CHECK-NEXT:    ret i1 false
+;
+  %can_load = call i1 @llvm.can.load.speculatively.p0(ptr %ptr, i64 32)
+  ret i1 %can_load
+}
+
+; Size > 16 - returns false (may cross MTE tag granule boundary)
+define i1 @can_load_speculatively_64(ptr %ptr) {
+; CHECK-LABEL: @can_load_speculatively_64(
+; CHECK-NEXT:    ret i1 false
+;
+  %can_load = call i1 @llvm.can.load.speculatively.p0(ptr %ptr, i64 64)
+  ret i1 %can_load
+}
+
+; Test with address space
+define i1 @can_load_speculatively_addrspace1(ptr addrspace(1) %ptr) {
+; CHECK-LABEL: @can_load_speculatively_addrspace1(
+; CHECK-NEXT:    ret i1 false
+;
+  %can_load = call i1 @llvm.can.load.speculatively.p1(ptr addrspace(1) %ptr, i64 16)
+  ret i1 %can_load
+}
+
+; Test size 8 (within limit, power-of-2)
+define i1 @can_load_speculatively_8(ptr %ptr) {
+; CHECK-LABEL: @can_load_speculatively_8(
+; CHECK-NEXT:    [[TMP1:%.*]] = ptrtoint ptr [[PTR:%.*]] to i64
+; CHECK-NEXT:    [[TMP2:%.*]] = and i64 [[TMP1]], 7
+; CHECK-NEXT:    [[TMP3:%.*]] = icmp eq i64 [[TMP2]], 0
+; CHECK-NEXT:    ret i1 [[TMP3]]
+;
+  %can_load = call i1 @llvm.can.load.speculatively.p0(ptr %ptr, i64 8)
+  ret i1 %can_load
+}
+
+; Test with runtime size - checks size <= 16 and alignment
+define i1 @can_load_speculatively_runtime(ptr %ptr, i64 %size) {
----------------
fhahn wrote:

Yep, added a number of tests with known alignments. Currently they don't get simplified by the TLI hook. 

The generated IR would get constant folded by InstCombine, but at this point we are too late, and it seems the backend passes won't simplify it; not sure what the best solution would be, adding the fold directly to TLI doesn't seem great.

https://github.com/llvm/llvm-project/pull/179642